DBConnection behaviour (db_connection v2.3.1) 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

conn()

cursor()

option()

params()

query()

result()

start_option()

status()

t()

Run or transaction connection reference.

Functions

__using__(_)

Use DBConnection to set the behaviour.

child_spec(conn_mod, opts)

Creates a supervisor child specification for a pool of connections.

close(conn, query, opts \\ [])

Close a prepared query on a database connection and return {:ok, result} on success or {:error, exception} on error.

close!(conn, query, opts \\ [])

Close a prepared query on a database connection and return the result. Raises an exception on error.

execute(conn, query, params, opts \\ [])

Execute a prepared query with a database connection and return {:ok, query, result} on success or {:error, exception} if there was an error.

execute!(conn, query, params, opts \\ [])

Execute a prepared query with a database connection and return the result. Raises an exception on error.

prepare(conn, query, opts \\ [])

Prepare a query with a database connection for later execution.

prepare!(conn, query, opts \\ [])

Prepare a query with a database connection and return the prepared query. An exception is raised on error.

prepare_execute(conn, query, params, opts \\ [])

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_execute!(conn, query, params, opts \\ [])

Prepare a query and execute it with a database connection and return both the prepared query and result. An exception is raised on error.

prepare_stream(conn, query, params, opts \\ [])

Create a stream that will prepare a query, execute it and stream results using a cursor.

reduce(stream, acc, fun)

Reduces a previously built stream or prepared stream.

rollback(conn, reason)

Rollback a database transaction and release lock on connection.

run(conn, fun, opts \\ [])

Acquire a lock on a connection and run a series of requests on it.

start_link(conn_mod, opts)

Starts and links to a database connection process.

status(conn, opts \\ [])

Return the transaction status of a connection.

stream(conn, query, params, opts \\ [])

Create a stream that will execute a prepared query and stream results using a cursor.

transaction(conn, fun, opts \\ [])

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

checkin(state)

Checks in the state to the connection process. Return {:ok, state} to allow the checkin or {:disconnect, exception, state} to disconnect.

checkout(state)

Checkouts the state from the connection process. Return {:ok, state} to allow the checkout or {:disconnect, exception, state} to disconnect.

connect(opts)

Connect to the database. Return {:ok, state} on success or {:error, exception} on failure.

disconnect(err, state)

Disconnect from the database. Return :ok.

handle_begin(opts, state)

Handle the beginning of a transaction.

handle_close(query, opts, state)

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_commit(opts, state)

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.

handle_deallocate(query, cursor, opts, state)

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.

handle_declare(query, params, opts, state)

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.

handle_execute(query, params, opts, state)

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.

handle_fetch(query, cursor, opts, state)

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_prepare(query, opts, state)

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.

handle_rollback(opts, state)

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_status(opts, state)

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.

ping(state)

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

conn()

Specs

conn() :: GenServer.server() | t()

cursor()

Specs

cursor() :: any()

option()

Specs

option() ::
  {:log,
   (DBConnection.LogEntry.t() -> any()) | {module(), atom(), [any()]} | nil}
  | {:queue, boolean()}
  | {:timeout, timeout()}
  | {:deadline, integer() | nil}

params()

Specs

params() :: any()

query()

Specs

query() :: DBConnection.Query.t()

result()

Specs

result() :: any()

start_option()

Specs

start_option() ::
  {:after_connect, (t() -> any()) | {module(), atom(), [any()]} | nil}
  | {:after_connect_timeout, timeout()}
  | {:connection_listeners, [Process.dest()] | nil}
  | {: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()}

status()

Specs

status() :: :idle | :transaction | :error

t()

Specs

t() :: %DBConnection{conn_mode: term(), conn_ref: reference(), pool_ref: any()}

Run or transaction connection reference.

Link to this section Functions

using(_)

(macro)

Use DBConnection to set the behaviour.

child_spec(conn_mod, opts)

Specs

child_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 \\ [])

Specs

close(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" in start_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. See System 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} with DBConnection.LogEntry.t/0 prepended to args or nil. See DBConnection.LogEntry (default: nil)

The pool and connection module may support other options. All options are passed to handle_close/3.

See prepare/3.

close!(conn, query, opts \\ [])

Specs

close!(conn(), query(), opts :: Keyword.t()) :: result()

Close a prepared query on a database connection and return the result. Raises an exception on error.

See close/3.

execute(conn, query, params, opts \\ [])

Specs

execute(conn(), query(), params(), opts :: 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" in start_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. See System 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} with DBConnection.LogEntry.t/0 prepended to args or nil. See DBConnection.LogEntry (default: nil)

The pool and connection module may support other options. All options are passed to handle_execute/4.

See prepare/3.

execute!(conn, query, params, opts \\ [])

Specs

execute!(conn(), query(), params(), opts :: Keyword.t()) :: result()

Execute a prepared query with a database connection and return the result. Raises an exception on error.

See execute/4

prepare(conn, query, opts \\ [])

Specs

prepare(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" in start_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. See System 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} with DBConnection.LogEntry.t/0 prepended to args or nil. See DBConnection.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!(conn, query, opts \\ [])

Specs

prepare!(conn(), query(), opts :: Keyword.t()) :: query()

Prepare a query with a database connection and return the prepared query. An exception is raised on error.

See prepare/3.

prepare_execute(conn, query, params, opts \\ [])

Specs

prepare_execute(conn(), query(), params(), 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" in start_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. See System 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} with DBConnection.LogEntry.t/0 prepended to args or nil. See DBConnection.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_execute!(conn, query, params, opts \\ [])

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 \\ [])

Specs

prepare_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" in start_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. See System 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} with DBConnection.LogEntry.t/0 prepended to args or nil. See DBConnection.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)

reduce(stream, acc, fun)

Reduces a previously built stream or prepared stream.

rollback(conn, reason)

Specs

rollback(t(), reason :: any()) :: no_return()

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)

run(conn, fun, opts \\ [])

Specs

run(conn(), (t() -> result), opts :: 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.

Options

:queue - Whether to block waiting in an internal queue for the connection's state (boolean, default: true). See "Queue config" in start_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. See System 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)

Specs

start_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 to args or nil where only returned options are passed to connect callback (default: nil)
:after_connect - A function to run on connect using run/3, either a 1-arity fun, {module, function, args} with DBConnection.t/0 prepended to args or nil (default: nil)
:after_connect_timeout - The maximum time allowed to perform function specified by :after_connect option (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 :name option in GenServer.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. Defaults to 1000ms.
: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 the Supervisor 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.

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}

status(conn, opts \\ [])

Specs

status(conn(), opts :: Keyword.t()) :: status()

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 \\ [])

Specs

stream(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" in start_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. See System 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} with DBConnection.LogEntry.t/0 prepended to args or nil. See DBConnection.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)

transaction(conn, fun, opts \\ [])

Specs

transaction(conn(), (t() -> result), opts :: 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}.

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" in start_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. See System 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} with DBConnection.LogEntry.t/0 prepended to args or nil. See DBConnection.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)

Specs

checkin(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)

Specs

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 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)

Specs

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.

disconnect(err, state)

Specs

disconnect(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_begin(opts, state)

Specs

handle_begin(opts :: Keyword.t(), state :: any()) ::
  {:ok, result(), new_state :: any()}
  | {status(), new_state :: any()}
  | {:disconnect, Exception.t(), new_state :: any()}

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.