[[index-modules-translog]]
== Translog

Changes to Lucene are only persisted to disk during a Lucene commit,
which is a relatively heavy operation and so cannot be performed after every
index or delete operation. Changes that happen after one commit and before another
will be lost in the event of process exit or HW failure.

To prevent this data loss, each shard has a _transaction log_ or write ahead
log associated with it. Any index or delete operation is written to the
translog after being processed by the internal Lucene index.

In the event of a crash, recent transactions can be replayed from the
transaction log when the shard recovers.

An Elasticsearch flush is the process of performing a Lucene commit and
starting a new translog. It is done automatically in the background in order
to make sure the transaction log doesn't grow too large, which would make
replaying its operations take a considerable amount of time during recovery.
It is also exposed through an API, though its rarely needed to be performed
manually.

[float]
=== Flush settings

The following <<indices-update-settings,dynamically updatable>> settings
control how often the in-memory buffer is flushed to disk:

`index.translog.flush_threshold_size`::

Once the translog hits this size, a flush will happen. Defaults to `512mb`.

[float]
=== Translog settings

The data in the transaction log is only persisted to disk when the translog is
++fsync++ed and committed.  In the event of hardware failure, any data written
since the previous translog commit will be lost.

By default, Elasticsearch ++fsync++s and commits the translog every 5 seconds
and at the end of every <<docs-index_,index>>, <<docs-delete,delete>>,
<<docs-update,update>>, or  <<docs-bulk,bulk>> request.  In fact, Elasticsearch
will only report success of an index, delete, update, or bulk request to the
client after the transaction log has been successfully ++fsync++ed and committed
on the primary and on every allocated replica.

The following <<indices-update-settings,dynamically updatable>> per-index settings
control the behaviour of the transaction log:

`index.translog.sync_interval`::

How often the translog is ++fsync++ed to disk and committed, regardless of
write operations. Defaults to `5s`.

`index.translog.durability`::
+
--

Whether or not to `fsync` and commit the translog after every index, delete,
update, or bulk request.  This setting accepts the following parameters:

`request`::

    (default) `fsync` and commit after every request. In the event
    of hardware failure, all acknowledged writes will already have been
    committed to disk.

`async`::

    `fsync` and commit in the background every `sync_interval`. In
    the event of hardware failure, all acknowledged writes since the last
    automatic commit will be discarded.
--

`index.translog.fs.type`::
+
--

Whether to buffer writes to the transaction log in memory or not.  This
setting accepts the following parameters:

`buffered`::

    (default) Translog writes first go to a 64kB buffer in memory,
    and are only written to the disk when the buffer is full, or when an
    `fsync` is triggered by a write request or the `sync_interval`.

`simple`::

    Translog writes are written to the file system immediately, without
    buffering.  However, these writes will only be persisted to disk when an
    `fsync` and commit is triggered by a write request or the `sync_interval`.

--