db_connection v2.2.0 DBConnection behaviour View Source
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.
Link to this section Summary
Types
Run or transaction connection reference.
Functions
Use DBConnection
to set the behaviour.
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.
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}
.
Callbacks
Checks in the state to the connection process. Return {:ok, state}
to allow the checkin or {:disconnect, exception, state}
to disconnect.
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
,
{:error, exception, state}
(deprecated) to error and no longer be inside
transaction, 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
,
{:error, exception, state}
(deprecated) to
error and no longer be inside transaction, 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.
Link to this section Types
start_option()
View Sourcestart_option() :: {:after_connect, (t() -> any()) | {module(), atom(), [any()]} | nil} | {:after_connect_timeout, timeout()} | {: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()} | {: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()}
Run or transaction connection reference.
Link to this section Functions
Use DBConnection
to set the behaviour.
child_spec(conn_mod, opts)
View Sourcechild_spec(module(), opts :: Keyword.t()) :: :supervisor.child_spec()
Creates a supervisor child specification for a pool of connections.
See start_link/2
for options.
close(conn, query, opts \\ [])
View Sourceclose(conn(), query(), opts :: 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/2
docs:timeout
- The maximum time that the caller is allowed to perform this operation (default:15_000
):deadline
- If set, overrides:timeout
option and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystem
module 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/0
prepended toargs
ornil
. SeeDBConnection.LogEntry
(default:nil
)
The pool and connection module may support other options. All options
are passed to handle_close/3
.
See prepare/3
.
Close a prepared query on a database connection and return the result. Raises an exception on error.
See close/3
.
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/2
docs:timeout
- The maximum time that the caller is allowed to perform this operation (default:15_000
):deadline
- If set, overrides:timeout
option and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystem
module 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/0
prepended toargs
ornil
. 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
prepare(conn, query, opts \\ [])
View Sourceprepare(conn(), query(), opts :: 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/2
docs:timeout
- The maximum time that the caller is allowed to perform this operation (default:15_000
):deadline
- If set, overrides:timeout
option and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystem
module 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/0
prepended toargs
ornil
. 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)
Prepare a query with a database connection and return the prepared query. An exception is raised on error.
See prepare/3
.
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/2
docs:timeout
- The maximum time that the caller is allowed to perform this operation (default:15_000
):deadline
- If set, overrides:timeout
option and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystem
module 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/0
prepended toargs
ornil
. 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
.
prepare_stream(conn, query, params, opts \\ [])
View Sourceprepare_stream(t(), query(), params(), opts :: 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/2
docs:timeout
- The maximum time that the caller is allowed to perform this operation (default:15_000
):deadline
- If set, overrides:timeout
option and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystem
module 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/0
prepended toargs
ornil
. 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)
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
.
Options
:queue
- Whether to block waiting in an internal queue for the connection's state (boolean, default:true
). See "Queue config" instart_link/2
docs:timeout
- The maximum time that the caller is allowed to perform this operation (default:15_000
):deadline
- If set, overrides:timeout
option and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystem
module 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)
start_link(conn_mod, opts)
View Sourcestart_link(module(), opts :: 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,:stop
for no backoff and to stop,:exp
for exponential,:rand
for random and:rand_exp
for 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 toargs
ornil
where only returned options are passed to connect callback (default:nil
):after_connect
- A function to run on connect usingrun/3
, either a 1-arity fun,{module, function, args}
withDBConnection.t/0
prepended toargs
ornil
(default:nil
):after_connect_timeout
- The maximum time allowed to perform function specified by:after_connect
option (default:15_000
):name
- A name to register the started process (see the:name
option inGenServer.start_link/3
):pool
- Chooses the pool to be started:pool_size
- Chooses the size of the pool:idle_interval
- Controls the frequency we ping the database when the connection is idle:queue_target
and:queue_interval
- See "Queue config" below:max_restarts
and:max_seconds
- Configures the:max_restarts
and:max_seconds
for the connection pool supervisor (see theSupervisor
docs):show_sensitive_data_on_connection_error
- By default,DBConnection
hides 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_target
in milliseconds, defaults to 50ms:queue_interval
in 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.
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
stream(conn, query, params, opts \\ [])
View Sourcestream(t(), query(), params(), opts :: 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/2
docs:timeout
- The maximum time that the caller is allowed to perform this operation (default:15_000
):deadline
- If set, overrides:timeout
option and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystem
module 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/0
prepended toargs
ornil
. 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)
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/2
docs:timeout
- The maximum time that the caller is allowed to perform this operation (default:15_000
):deadline
- If set, overrides:timeout
option and specifies absolute monotonic time in milliseconds by which caller must perform operation. SeeSystem
module 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/0
prepended toargs
ornil
. 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)
Link to this section Callbacks
checkin(state)
View Sourcecheckin(state :: any()) :: {:ok, new_state :: any()} | {:disconnect, Exception.t(), new_state :: any()}
Checks in the state to the connection process. Return {:ok, state}
to allow the checkin or {:disconnect, exception, state}
to disconnect.
This callback is called when the control of the state is passed back
to the connection process. It should reverse any changes to the
connection state made in checkout/1
.
This callback is called in the connection process.
checkout(state)
View Sourcecheckout(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 when the control of the state is passed to
another process. checkin/1
is called with the new state when control
is returned to the connection process.
This callback is called in the connection process.
connect(opts)
View Sourceconnect(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.
disconnect(err, state)
View Sourcedisconnect(err :: Exception.t(), state :: any()) :: :ok
Disconnect from the database. Return :ok
.
The exception as first argument is the exception from a :disconnect
3-tuple returned by a previous callback.
If the state is controlled by a client and it exits or takes too long
to process a request the state will be last known state. In these
cases the exception will be a DBConnection.ConnectionError
.
This callback is called in the connection process.
Handle the beginning of a transaction.
Return {:ok, result, state}
to continue, {status, state}
to notify caller
that the transaction can not begin due to the transaction status status
,
{:error, exception, state}
(deprecated) to error without beginning the
transaction, 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.
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.
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
,
{:error, exception, state}
(deprecated) to error and no longer be inside
transaction, 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.
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.
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.
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.
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.
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.
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
,
{:error, exception, state}
(deprecated) to
error and no longer be inside transaction, 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.
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
.
ping(state)
View Sourceping(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
option. This function
can be called whether the connection is checked in or checked out.
This callback is called in the connection process.