db_connection v2.0.2 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 a period of time the connection will trigger an idle timeout and the database can be pinged 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 prepared or encoded and results decoding without blocking the connection or pool.

Link to this section Summary

Types

conn()

cursor()

params()

query()

result()

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 the result. Raises an exception on error

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

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

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

Execute a prepared query with 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

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

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

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

Prepare a query with a database connection for later execution

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_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_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, cursor, state} to start a cursor for a stream and continue, {: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, result, state} to return the result result and continue, {: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()

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

cursor()

cursor() :: any()

params()

params() :: any()

query()

query() :: any()

result()

result() :: any()

status()

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

t()

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)

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 \\ [])

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.

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

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)
: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.

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

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

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

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)
: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.

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

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(conn, query, opts \\ [])

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)
: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_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_execute(conn, query, params, opts \\ [])

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)
: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_stream(conn, query, params, opts \\ [])

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)
: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)

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 \\ [])

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)

The pool may support other options.

Example

{:ok, res} = DBConnection.run(conn, fn conn ->
  DBConnection.execute!(conn, query, [])
end)

start_link(conn_mod, opts)

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)
: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
:queue_target and :queue_interval - See “Queue config” below

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 microseconds, defaults to 50
:queue_interval in microseconds, defaults to 1000

Our goal is to stay under :queue_target for :queue_interval. In case we can’t reach that, then we double the :queue_target. If we go above that, then we start dropping messages.

For example, by default our queue time is 50ms. If we stay above 50ms for a whole secnod, we double the target to 100ms and we start dropping messages once it 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.

status(conn, opts \\ [])

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 assumption based.

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 \\ [])

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)
: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 \\ [])

transaction(conn(), (conn() -> 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)
: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)

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)

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)

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)

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)

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.