View Source DBConnection behaviour (db_connection v2.6.0)
A behaviour module for implementing efficient database connection client processes, pools and transactions.
DBConnection handles callbacks differently to most behaviours. Some
callbacks will be called in the calling process, with the state
copied to and from the calling process. This is useful when the data
for a request is large and means that a calling process can interact
with a socket directly.
A side effect of this is that query handling can be written in a simple blocking fashion, while the connection process itself will remain responsive to OTP messages and can enqueue and cancel queued requests.
If a request or series of requests takes too long to handle in the client process a timeout will trigger and the socket can be cleanly disconnected by the connection process.
If a calling process waits too long to start its request it will timeout and its request will be cancelled. This prevents requests building up when the database can not keep up.
If no requests are received for an idle interval, the pool will ping all stale connections which can then ping the database to keep the connection alive.
Should the connection be lost, attempts will be made to reconnect with (configurable) exponential random backoff to reconnect. All state is lost when a connection disconnects but the process is reused.
The DBConnection.Query protocol provide utility functions so that
queries can be encoded and decoded without blocking the connection or pool.
Connection pools
DBConnection connections support using different pools via the :pool option
passed to start_link/2. The default pool is DBConnection.ConnectionPool.
Another supported pool that is commonly used for tests is DBConnection.Ownership.
For now, using custom pools is not supported since the API for pools is not public.
Errors
Most functions in this module raise a DBConnection.ConnectionError exception
when failing to check out a connection from the pool.
Summary
Types
An option you can pass to DBConnection functions.
An option you can pass to DBConnection functions (deprecated).
Run or transaction connection reference.
Callbacks
Checkouts the state from the connection process. Return {:ok, state}
to allow the checkout or {:disconnect, exception, state} to disconnect.
Connect to the database. Return {:ok, state} on success or
{:error, exception} on failure.
Disconnect from the database. Return :ok.
Handle the beginning of a transaction.
Close a query prepared by handle_prepare/3 with the database. Return
{:ok, result, state} on success and to continue,
{:error, exception, state} to return an error and continue, or
{:disconnect, exception, state} to return an error and disconnect.
Handle committing a transaction. Return {:ok, result, state} on successfully
committing transaction, {status, state} to notify caller that the
transaction can not commit due to the transaction status status,
or {:disconnect, exception, state} to error and disconnect.
Deallocate a cursor declared by handle_declare/4 with the database. Return
{:ok, result, state} on success and to continue,
{:error, exception, state} to return an error and continue, or
{:disconnect, exception, state} to return an error and disconnect.
Declare a cursor using a query prepared by handle_prepare/3. Return
{:ok, query, cursor, state} to return altered query query and cursor
cursor for a stream and continue, {:error, exception, state} to return an
error and continue or {:disconnect, exception, state} to return an error
and disconnect.
Execute a query prepared by handle_prepare/3. Return
{:ok, query, result, state} to return altered query query and result
result and continue, {:error, exception, state} to return an error and
continue or {:disconnect, exception, state} to return an error and
disconnect.
Fetch the next result from a cursor declared by handle_declare/4. Return
{:cont, result, state} to return the result result and continue using
cursor, {:halt, result, state} to return the result result and close the
cursor, {:error, exception, state} to return an error and close the
cursor, {:disconnect, exception, state} to return an error and disconnect.
Handle rolling back a transaction. Return {:ok, result, state} on successfully
rolling back transaction, {status, state} to notify caller that the
transaction can not rollback due to the transaction status status or
{:disconnect, exception, state} to error and disconnect.
Handle getting the transaction status. Return {:idle, state} if outside a
transaction, {:transaction, state} if inside a transaction,
{:error, state} if inside an aborted transaction, or
{:disconnect, exception, state} to error and disconnect.
Called when the connection has been idle for a period of time. Return
{:ok, state} to continue or {:disconnect, exception, state} to
disconnect.
Functions
Use DBConnection to set the behaviour.
Returns the names of all possible options that you can pass to most functions in this module.
Returns the names of all possible options that you can pass to start_link/2.
Creates a supervisor child specification for a pool of connections.
Close a prepared query on a database connection and return {:ok, result} on
success or {:error, exception} on error.
Close a prepared query on a database connection and return the result. Raises an exception on error.
Returns connection module used by the given connection pool.
Forces all connections in the pool to disconnect within the given interval in milliseconds.
Execute a prepared query with a database connection and return
{:ok, query, result} on success or {:error, exception} if there was an error.
Execute a prepared query with a database connection and return the result. Raises an exception on error.
Prepare a query with a database connection for later execution.
Prepare a query with a database connection and return the prepared query. An exception is raised on error.
Prepare a query and execute it with a database connection and return both the
prepared query and the result, {:ok, query, result} on success or
{:error, exception} if there was an error.
Prepare a query and execute it with a database connection and return both the prepared query and result. An exception is raised on error.
Create a stream that will prepare a query, execute it and stream results using a cursor.
Reduces a previously built stream or prepared stream.
Rollback a database transaction and release lock on connection.
Acquire a lock on a connection and run a series of requests on it.
Starts and links to a database connection process.
Return the transaction status of a connection.
Create a stream that will execute a prepared query and stream results using a cursor.
Acquire a lock on a connection and run a series of requests inside a
transaction. The result of the transaction fun is return inside an :ok
tuple: {:ok, result}.
Types
@type conn() :: GenServer.server() | t()
@type connection_option() :: {:log, (DBConnection.LogEntry.t() -> any()) | {module(), atom(), [any()]} | nil} | {:queue, boolean()} | {:timeout, timeout()} | {:deadline, integer() | nil}
An option you can pass to DBConnection functions.
@type cursor() :: any()
@type option() :: connection_option()
An option you can pass to DBConnection functions (deprecated).
Deprecated
This option is deprecated since v2.6.0. Use
connection_option/0instead.
@type params() :: any()
@type query() :: DBConnection.Query.t()
@type result() :: any()
@type start_option() :: {:after_connect, (t() -> any()) | {module(), atom(), [any()]} | nil} | {:after_connect_timeout, timeout()} | {:connection_listeners, [Process.dest()] | nil | {[Process.dest()], any()}} | {:backoff_max, non_neg_integer()} | {:backoff_min, non_neg_integer()} | {:backoff_type, :stop | :exp | :rand | :rand_exp} | {:configure, (keyword() -> keyword()) | {module(), atom(), [any()]} | nil} | {:idle_interval, non_neg_integer()} | {:idle_limit, non_neg_integer()} | {:max_restarts, non_neg_integer()} | {:max_seconds, pos_integer()} | {:name, GenServer.name()} | {:pool, module()} | {:pool_size, pos_integer()} | {:queue_interval, non_neg_integer()} | {:queue_target, non_neg_integer()} | {:show_sensitive_data_on_connection_error, boolean()}
@type status() :: :idle | :transaction | :error
Run or transaction connection reference.
Callbacks
@callback checkout(state :: any()) :: {:ok, new_state :: any()} | {:disconnect, Exception.t(), new_state :: any()}
Checkouts the state from the connection process. Return {:ok, state}
to allow the checkout or {:disconnect, exception, state} to disconnect.
This callback is called immediately after the connection is established
and the state is never effetively checked in again. That's because
DBConnection keeps the connection state in an ETS table that is moved
between the different clients checking out connections. There is no
checkin callback. The state is only handed back to the connection
process during pings and (re)connects.
This callback is called in the connection process.
@callback connect(opts :: Keyword.t()) :: {:ok, state :: any()} | {:error, Exception.t()}
Connect to the database. Return {:ok, state} on success or
{:error, exception} on failure.
If an error is returned it will be logged and another connection attempt will be made after a backoff interval.
This callback is called in the connection process.
@callback disconnect(err :: Exception.t(), state :: any()) :: :ok
Disconnect from the database. Return :ok.
This callback is called from the connection process. The first argument is
either the exception from a :disconnect 3-tuple returned by a previous
callback or an exception generated by the connection process.
If the state is controlled by a client and it exits or times out while
processing a request, the last known state will be sent and the exception
will be a DBConnection.ConnectionError.
When the connection is stopped, this callback will be invoked from terminate.
The last known state will be sent and the exception will be a DBConnection.ConnectionError
containing the reason for the exit. To have the same happen on unexpected
shutdowns, you may trap exits from the connect callback.
@callback handle_begin(opts :: Keyword.t(), state :: any()) :: {:ok, result(), new_state :: any()} | {:ok, query(), result(), new_state :: any()} | {status(), new_state :: any()} | {:disconnect, Exception.t(), new_state :: any()}
Handle the beginning of a transaction.
Return {:ok, result, state}/{:ok, query, result, state} to continue,
{status, state} to notify caller that the transaction can not begin due
to the transaction status status, or {:disconnect, exception, state}
to error and disconnect. If {:ok, query, result, state} is returned,
the query will be used to log the begin command. Otherwise, it will be
logged as begin.
A callback implementation should only return status if it
can determine the database's transaction status without side effect.
This callback is called in the client process.
@callback handle_close(query(), opts :: Keyword.t(), state :: any()) :: {:ok, result(), new_state :: any()} | {:error | :disconnect, Exception.t(), new_state :: any()}
Close a query prepared by handle_prepare/3 with the database. Return
{:ok, result, state} on success and to continue,
{:error, exception, state} to return an error and continue, or
{:disconnect, exception, state} to return an error and disconnect.
This callback is called in the client process.
@callback handle_commit(opts :: Keyword.t(), state :: any()) :: {:ok, result(), new_state :: any()} | {status(), new_state :: any()} | {:disconnect, Exception.t(), new_state :: any()}
Handle committing a transaction. Return {:ok, result, state} on successfully
committing transaction, {status, state} to notify caller that the
transaction can not commit due to the transaction status status,
or {:disconnect, exception, state} to error and disconnect.
A callback implementation should only return status if it
can determine the database's transaction status without side effect.
This callback is called in the client process.
@callback handle_deallocate(query(), cursor(), opts :: Keyword.t(), state :: any()) :: {:ok, result(), new_state :: any()} | {:error | :disconnect, Exception.t(), new_state :: any()}
Deallocate a cursor declared by handle_declare/4 with the database. Return
{:ok, result, state} on success and to continue,
{:error, exception, state} to return an error and continue, or
{:disconnect, exception, state} to return an error and disconnect.
This callback is called in the client process.
@callback handle_declare(query(), params(), opts :: Keyword.t(), state :: any()) :: {:ok, query(), cursor(), new_state :: any()} | {:error | :disconnect, Exception.t(), new_state :: any()}
Declare a cursor using a query prepared by handle_prepare/3. Return
{:ok, query, cursor, state} to return altered query query and cursor
cursor for a stream and continue, {:error, exception, state} to return an
error and continue or {:disconnect, exception, state} to return an error
and disconnect.
This callback is called in the client process.
@callback handle_execute(query(), params(), opts :: Keyword.t(), state :: any()) :: {:ok, query(), result(), new_state :: any()} | {:error | :disconnect, Exception.t(), new_state :: any()}
Execute a query prepared by handle_prepare/3. Return
{:ok, query, result, state} to return altered query query and result
result and continue, {:error, exception, state} to return an error and
continue or {:disconnect, exception, state} to return an error and
disconnect.
This callback is called in the client process.
@callback handle_fetch(query(), cursor(), opts :: Keyword.t(), state :: any()) :: {:cont | :halt, result(), new_state :: any()} | {:error | :disconnect, Exception.t(), new_state :: any()}
Fetch the next result from a cursor declared by handle_declare/4. Return
{:cont, result, state} to return the result result and continue using
cursor, {:halt, result, state} to return the result result and close the
cursor, {:error, exception, state} to return an error and close the
cursor, {:disconnect, exception, state} to return an error and disconnect.
This callback is called in the client process.
@callback handle_prepare(query(), opts :: Keyword.t(), state :: any()) :: {:ok, query(), new_state :: any()} | {:error | :disconnect, Exception.t(), new_state :: any()}
Prepare a query with the database. Return {:ok, query, state} where
query is a query to pass to execute/4 or close/3,
{:error, exception, state} to return an error and continue or
{:disconnect, exception, state} to return an error and disconnect.
This callback is intended for cases where the state of a connection is needed to prepare a query and/or the query can be saved in the database to call later.
This callback is called in the client process.
@callback handle_rollback(opts :: Keyword.t(), state :: any()) :: {:ok, result(), new_state :: any()} | {status(), new_state :: any()} | {:disconnect, Exception.t(), new_state :: any()}
Handle rolling back a transaction. Return {:ok, result, state} on successfully
rolling back transaction, {status, state} to notify caller that the
transaction can not rollback due to the transaction status status or
{:disconnect, exception, state} to error and disconnect.
A callback implementation should only return status if it
can determine the database' transaction status without side effect.
This callback is called in the client and connection process.
@callback handle_status(opts :: Keyword.t(), state :: any()) :: {status(), new_state :: any()} | {:disconnect, Exception.t(), new_state :: any()}
Handle getting the transaction status. Return {:idle, state} if outside a
transaction, {:transaction, state} if inside a transaction,
{:error, state} if inside an aborted transaction, or
{:disconnect, exception, state} to error and disconnect.
If the callback returns a :disconnect tuples then status/2 will return
:error.
@callback ping(state :: any()) :: {:ok, new_state :: any()} | {:disconnect, Exception.t(), new_state :: any()}
Called when the connection has been idle for a period of time. Return
{:ok, state} to continue or {:disconnect, exception, state} to
disconnect.
This callback is called if no callbacks have been called after the
idle timeout and a client process is not using the state. The idle
timeout can be configured by the :idle_interval and :idle_limit
options. This function can be called whether the connection is checked
in or checked out.
This callback is called in the connection process.
Functions
Use DBConnection to set the behaviour.
@spec available_connection_options() :: [atom(), ...]
Returns the names of all possible options that you can pass to most functions in this module.
This is mostly useful for library authors that base their library on top of
DBConnection, since they can use the return value of this function to perform
validation on options only passing down these options to DBConnection.
See also connection_option/0.
@spec available_start_options() :: [atom(), ...]
Returns the names of all possible options that you can pass to start_link/2.
This is mostly useful for library authors that base their library on top of
DBConnection, since they can use the return value of this function to perform
validation on options only passing down these options to DBConnection.
See also start_option/0.
@spec child_spec(module(), [start_option()] | Keyword.t()) :: :supervisor.child_spec()
Creates a supervisor child specification for a pool of connections.
See start_link/2 for options.
@spec close(conn(), query(), [connection_option()] | Keyword.t()) :: {:ok, result()} | {:error, Exception.t()}
Close a prepared query on a database connection and return {:ok, result} on
success or {:error, exception} on error.
This function should be used to free resources held by the connection process and/or the database server.
Options
:queue- Whether to block waiting in an internal queue for the connection's state (boolean, default:true). See "Queue config" instart_link/2docs:timeout- The maximum time that the caller is allowed to perform this operation (default:15_000):deadline- If set, overrides:timeoutoption and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystemmodule documentation for more information on monotonic time (default:nil):log- A function to log information about a call, either a 1-arity fun,{module, function, args}withDBConnection.LogEntry.t/0prepended toargsornil. SeeDBConnection.LogEntry(default:nil)
The pool and connection module may support other options. All options
are passed to handle_close/3.
See prepare/3.
@spec close!(conn(), query(), [connection_option()] | Keyword.t()) :: result()
Close a prepared query on a database connection and return the result. Raises an exception on error.
See close/3.
Returns connection module used by the given connection pool.
When given a process that is not a connection pool, returns an :error.
@spec disconnect_all(conn(), non_neg_integer(), [connection_option()] | Keyword.t()) :: :ok
Forces all connections in the pool to disconnect within the given interval in milliseconds.
Once this function is called, the pool will disconnect all of its connections
as they are checked in or as they are pinged. Checked in connections will be
randomly disconnected within the given time interval. Pinged connections are
immediately disconnected - as they are idle (according to :idle_interval).
If the connection has a backoff configured (which is the case by default), disconnecting means an attempt at a new connection will be done immediately after, without starting a new process for each connection. However, if backoff has been disabled, the connection process will terminate. In such cases, disconnecting all connections may cause the pool supervisor to restart depending on the max_restarts/max_seconds configuration of the pool, so you will want to set those carefully.
@spec execute(conn(), query(), params(), [connection_option()] | Keyword.t()) :: {:ok, query(), result()} | {:error, Exception.t()}
Execute a prepared query with a database connection and return
{:ok, query, result} on success or {:error, exception} if there was an error.
If the query is not prepared on the connection an attempt may be made to prepare it and then execute again.
Options
:queue- Whether to block waiting in an internal queue for the connection's state (boolean, default:true). See "Queue config" instart_link/2docs:timeout- The maximum time that the caller is allowed to perform this operation (default:15_000):deadline- If set, overrides:timeoutoption and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystemmodule documentation for more information on monotonic time (default:nil):log- A function to log information about a call, either a 1-arity fun,{module, function, args}withDBConnection.LogEntry.t/0prepended toargsornil. SeeDBConnection.LogEntry(default:nil)
The pool and connection module may support other options. All options
are passed to handle_execute/4.
See prepare/3.
Execute a prepared query with a database connection and return the result. Raises an exception on error.
See execute/4
@spec prepare(conn(), query(), [connection_option()] | Keyword.t()) :: {:ok, query()} | {:error, Exception.t()}
Prepare a query with a database connection for later execution.
It returns {:ok, query} on success or {:error, exception} if there was
an error.
The returned query can then be passed to execute/4 and/or close/3
Options
:queue- Whether to block waiting in an internal queue for the connection's state (boolean, default:true). See "Queue config" instart_link/2docs:timeout- The maximum time that the caller is allowed to perform this operation (default:15_000):deadline- If set, overrides:timeoutoption and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystemmodule documentation for more information on monotonic time (default:nil):log- A function to log information about a call, either a 1-arity fun,{module, function, args}withDBConnection.LogEntry.t/0prepended toargsornil. SeeDBConnection.LogEntry(default:nil)
The pool and connection module may support other options. All options
are passed to handle_prepare/3.
Example
DBConnection.transaction(pool, fn conn ->
query = %Query{statement: "SELECT * FROM table"}
query = DBConnection.prepare!(conn, query)
try do
DBConnection.execute!(conn, query, [])
after
DBConnection.close(conn, query)
end
end)@spec prepare!(conn(), query(), [connection_option()] | Keyword.t()) :: query()
Prepare a query with a database connection and return the prepared query. An exception is raised on error.
See prepare/3.
@spec prepare_execute(conn(), query(), params(), [connection_option()] | Keyword.t()) :: {:ok, query(), result()} | {:error, Exception.t()}
Prepare a query and execute it with a database connection and return both the
prepared query and the result, {:ok, query, result} on success or
{:error, exception} if there was an error.
The returned query can be passed to execute/4 and close/3.
Options
:queue- Whether to block waiting in an internal queue for the connection's state (boolean, default:true). See "Queue config" instart_link/2docs:timeout- The maximum time that the caller is allowed to perform this operation (default:15_000):deadline- If set, overrides:timeoutoption and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystemmodule documentation for more information on monotonic time (default:nil):log- A function to log information about a call, either a 1-arity fun,{module, function, args}withDBConnection.LogEntry.t/0prepended toargsornil. SeeDBConnection.LogEntry(default:nil)
Example
query = %Query{statement: "SELECT id FROM table WHERE id=$1"}
{:ok, query, result} = DBConnection.prepare_execute(conn, query, [1])
{:ok, result2} = DBConnection.execute(conn, query, [2])
:ok = DBConnection.close(conn, query)Prepare a query and execute it with a database connection and return both the prepared query and result. An exception is raised on error.
See prepare_execute/4.
@spec prepare_stream(t(), query(), params(), [connection_option()] | Keyword.t()) :: DBConnection.PrepareStream.t()
Create a stream that will prepare a query, execute it and stream results using a cursor.
Options
:queue- Whether to block waiting in an internal queue for the connection's state (boolean, default:true). See "Queue config" instart_link/2docs:timeout- The maximum time that the caller is allowed to perform this operation (default:15_000):deadline- If set, overrides:timeoutoption and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystemmodule documentation for more information on monotonic time (default:nil):log- A function to log information about a call, either a 1-arity fun,{module, function, args}withDBConnection.LogEntry.t/0prepended toargsornil. SeeDBConnection.LogEntry(default:nil)
The pool and connection module may support other options. All options
are passed to handle_prepare/3, handle_close/3, handle_declare/4,
and handle_deallocate/4.
Example
{:ok, results} = DBConnection.transaction(conn, fn conn ->
query = %Query{statement: "SELECT id FROM table"}
stream = DBConnection.prepare_stream(conn, query, [])
Enum.to_list(stream)
end)Reduces a previously built stream or prepared stream.
Rollback a database transaction and release lock on connection.
When inside of a transaction/3 call does a non-local return, using a
throw/1 to cause the transaction to enter a failed state and the
transaction/3 call returns {:error, reason}. If transaction/3 calls are
nested the connection is marked as failed until the outermost transaction call
does the database rollback.
Example
{:error, :oops} = DBConnection.transaction(pool, fun(conn) ->
DBConnection.rollback(conn, :oops)
end)@spec run(conn(), (t() -> result), [connection_option()] | Keyword.t()) :: result when result: var
Acquire a lock on a connection and run a series of requests on it.
The return value of this function is the return value of fun.
To use the locked connection call the request with the connection
reference passed as the single argument to the fun. If the
connection disconnects all future calls using that connection
reference will fail.
run/3 and transaction/3 can be nested multiple times but a
transaction/3 call inside another transaction/3 will be treated
the same as run/3.
Checkout failures
If we cannot check out a connection from the pool, this function raises a
DBConnection.ConnectionErrorexception. If you want to handle these cases, you should rescueDBConnection.ConnectionErrorexceptions when usingrun/3.
Options
:queue- Whether to block waiting in an internal queue for the connection's state (boolean, default:true). See "Queue config" instart_link/2docs:timeout- The maximum time that the caller is allowed to perform this operation (default:15_000):deadline- If set, overrides:timeoutoption and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystemmodule documentation for more information on monotonic time (default:nil)
The pool may support other options.
Example
{:ok, res} = DBConnection.run(conn, fn conn ->
DBConnection.execute!(conn, query, [])
end)@spec start_link(module(), [start_option()] | Keyword.t()) :: GenServer.on_start()
Starts and links to a database connection process.
By default the DBConnection starts a pool with a single connection.
The size of the pool can be increased with :pool_size. A separate
pool can be given with the :pool option.
Options
:backoff_min- The minimum backoff interval (default:1_000):backoff_max- The maximum backoff interval (default:30_000):backoff_type- The backoff strategy,:stopfor no backoff and to stop,:expfor exponential,:randfor random and:rand_expfor random exponential (default::rand_exp):configure- A function to run before every connect attempt to dynamically configure the options, either a 1-arity fun,{module, function, args}with options prepended toargsornilwhere only returned options are passed to connect callback (default:nil). This function is called in the connection process.:after_connect- A function to run on connect usingrun/3, either a 1-arity fun,{module, function, args}withDBConnection.t/0prepended toargsornil(default:nil):after_connect_timeout- The maximum time allowed to perform function specified by:after_connectoption (default:15_000):connection_listeners- A list of process destinations to send notification messages whenever a connection is connected or disconnected. See "Connection listeners" below:name- A name to register the started process (see the:nameoption inGenServer.start_link/3):pool- Chooses the pool to be started (default:DBConnection.ConnectionPool). See "Connection pools".:pool_size- Chooses the size of the pool (default:1):idle_interval- Controls the frequency we check for idle connections in the pool. We then notify each idle connection to ping the database. In practice, the ping happens withinidle_interval <= ping < 2 * idle_interval. Defaults to 1000ms.:idle_limit- The number of connections to ping on each:idle_interval. Defaults to the pool size (all connections).:queue_targetand:queue_interval- See "Queue config" below:max_restartsand:max_seconds- Configures the:max_restartsand:max_secondsfor the connection pool supervisor (see theSupervisordocs). Typically speaking the connection process doesn't terminate, except due to faults in DBConnection. However, if backoff has been disabled, then they also terminate whenever a connection is disconnected (for instance, due to client or server errors):show_sensitive_data_on_connection_error- By default,DBConnectionhides all information during connection errors to avoid leaking credentials or other sensitive information. You can set this option if you wish to see complete errors and stacktraces during connection errors
Example
{:ok, conn} = DBConnection.start_link(mod, [idle_interval: 5_000])Queue config
Handling requests is done through a queue. When DBConnection is started, there are two relevant options to control the queue:
:queue_targetin milliseconds, defaults to 50ms:queue_intervalin milliseconds, defaults to 1000ms
Our goal is to wait at most :queue_target for a connection.
If all connections checked out during a :queue_interval takes
more than :queue_target, then we double the :queue_target.
If checking out connections take longer than the new target,
then we start dropping messages.
For example, by default our target is 50ms. If all connections checkouts take longer than 50ms for a whole second, we double the target to 100ms and we start dropping messages if the time to checkout goes above the new limit.
This allows us to better plan for overloads as we can refuse requests before they are sent to the database, which would otherwise increase the burden on the database, making the overload worse.
Connection listeners
The :connection_listeners option allows one or more processes to be notified
whenever a connection is connected or disconnected. A listener may be a remote
or local PID, a locally registered name, or a tuple in the form of
{registered_name, node} for a registered name at another node.
Each listener process may receive the following messages where pid
identifies the connection process:
{:connected, pid}{:disconnected, pid}
If the value of :connection_listeners is a tuple like {listeners, term}, then
the messages are these instead:
{:connected, pid, term}{:disconnected, pid, term}
Note the disconnected messages are not guaranteed to be delivered if the
pid for connection crashes. So it is recommended to monitor the connected
pid if you want to track all disconnections.
Here is an example of a :connection_listener implementation:
defmodule DBConnectionListener do
use GenServer
def start_link(opts) do
GenServer.start_link(__MODULE__, [], opts)
end
def get_notifications(pid) do
GenServer.call(pid, :read_state)
end
@impl true
def init(stack) when is_list(stack) do
{:ok, stack}
end
@impl true
def handle_call(:read_state, _from, state) do
{:reply, state, state}
end
@impl true
def handle_info({:connected, _pid} = msg, state) do
{:noreply, [msg | state]}
end
@impl true
def handle_info({_other_states, _pid} = msg, state) do
{:noreply, [msg | state]}
end
endYou can then start it, pass the PID in the connection_listeners
option on DBConnection.start_link/2 and, when needed, can query the notifications:
{:ok, pid} = DBConnectionListener.start_link([])
{:ok, _conn} = DBConnection.start_link(SomeModule, [connection_listeners: [pid]])
notifications = DBConnectionListener.get_notifications(pid)Tagging messages
If you pass {listeners, tag} as an option, you can specify an arbitrary tag term that will
be sent alongside all :connected/:disconnected messages. This is useful if you
want to track information about the pool a connection belongs to or any other information.
This feature is available since v2.6.0. Before this version :connection_listeners only
accepted a list of listener processes.
Telemetry
A [:db_connection, :connection_error] event is published whenever a
connection checkout receives a %DBConnection.ConnectionError{}.
Measurements:
:count- A fixed-value measurement which always measures 1.
Metadata
:error- TheDBConnection.ConnectionErrorstruct which triggered the event.:opts- All options given to the pool operation
Return the transaction status of a connection.
The callback implementation should return the transaction status according to the database, and not make assumptions based on client-side state.
This function will raise a DBConnection.ConnectionError when called inside a
deprecated transaction/3.
Options
See module documentation. The pool and connection module may support other
options. All options are passed to handle_status/2.
Example
# outside of the transaction, the status is `:idle`
DBConnection.status(conn) #=> :idle
DBConnection.transaction(conn, fn conn ->
DBConnection.status(conn) #=> :transaction
# run a query that will cause the transaction to rollback, e.g.
# uniqueness constraint violation
DBConnection.execute(conn, bad_query, [])
DBConnection.status(conn) #=> :error
end)
DBConnection.status(conn) #=> :idle@spec stream(t(), query(), params(), [connection_option()] | Keyword.t()) :: DBConnection.Stream.t()
Create a stream that will execute a prepared query and stream results using a cursor.
Options
:queue- Whether to block waiting in an internal queue for the connection's state (boolean, default:true). See "Queue config" instart_link/2docs:timeout- The maximum time that the caller is allowed to perform this operation (default:15_000):deadline- If set, overrides:timeoutoption and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystemmodule documentation for more information on monotonic time (default:nil):log- A function to log information about a call, either a 1-arity fun,{module, function, args}withDBConnection.LogEntry.t/0prepended toargsornil. SeeDBConnection.LogEntry(default:nil)
The pool and connection module may support other options. All options
are passed to handle_declare/4 and handle_deallocate/4.
Example
DBConnection.transaction(pool, fn conn ->
query = %Query{statement: "SELECT id FROM table"}
query = DBConnection.prepare!(conn, query)
try do
stream = DBConnection.stream(conn, query, [])
Enum.to_list(stream)
after
# Make sure query is closed!
DBConnection.close(conn, query)
end
end)@spec transaction(conn(), (t() -> result), [connection_option()] | Keyword.t()) :: {:ok, result} | {:error, reason :: any()} when result: var
Acquire a lock on a connection and run a series of requests inside a
transaction. The result of the transaction fun is return inside an :ok
tuple: {:ok, result}.
To use the locked connection call the request with the connection
reference passed as the single argument to the fun. If the
connection disconnects all future calls using that connection
reference will fail.
run/3 and transaction/3 can be nested multiple times. If a transaction is
rolled back or a nested transaction fun raises the transaction is marked as
failed. All calls except run/3, transaction/3, rollback/2, close/3 and
close!/3 will raise an exception inside a failed transaction until the outer
transaction call returns. All transaction/3 calls will return
{:error, :rollback} if the transaction failed or connection closed and
rollback/2 is not called for that transaction/3.
Options
:queue- Whether to block waiting in an internal queue for the connection's state (boolean, default:true). See "Queue config" instart_link/2docs:timeout- The maximum time that the caller is allowed to perform this operation (default:15_000):deadline- If set, overrides:timeoutoption and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystemmodule documentation for more information on monotonic time (default:nil):log- A function to log information about begin, commit and rollback calls made as part of the transaction, either a 1-arity fun,{module, function, args}withDBConnection.LogEntry.t/0prepended toargsornil. SeeDBConnection.LogEntry(default:nil)
The pool and connection module may support other options. All options
are passed to handle_begin/2, handle_commit/2 and
handle_rollback/2.
Example
{:ok, res} = DBConnection.transaction(conn, fn conn ->
DBConnection.execute!(conn, query, [])
end)