[role="xpack"]
[[transform-checkpoints]]
= How {transform} checkpoints work
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<titleabbrev>How checkpoints work</titleabbrev>
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Each time a {transform} examines the source indices and creates or updates the 
destination index, it generates a _checkpoint_.

If your {transform} runs only once, there is logically only one checkpoint. If 
your {transform} runs continuously, however, it creates checkpoints as it 
ingests and transforms new source data. The `sync` property of the {transform} 
configures checkpointing by specifying a time field.

To create a checkpoint, the {ctransform}:

. Checks for changes to source indices.
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Using a simple periodic timer, the {transform} checks for changes to the source 
indices. This check is done based on the interval defined in the transform's 
`frequency` property.
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If the source indices remain unchanged or if a checkpoint is already in progress
then it waits for the next timer.
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If changes are found a checkpoint is created.

. Identifies which entities and/or time buckets have changed.
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The {transform} searches to see which entities or time buckets have changed 
between the last and the new checkpoint. The {transform} uses the values to
synchronize the source and destination indices with fewer operations than a
full re-run.
 
. Updates the destination index (the {dataframe}) with the changes.
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The {transform} applies changes related to either new or changed entities or
time buckets to the destination index. The set of changes can be paginated. The
{transform} performs a composite aggregation similarly to the batch {transform} 
operation, however it also injects query filters based on the previous step to 
reduce the amount of work. After all changes have been applied, the checkpoint is 
complete.
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This checkpoint process involves both search and indexing activity on the
cluster. We have attempted to favor control over performance while developing
{transforms}. We decided it was preferable for the {transform} to take longer to 
complete, rather than to finish quickly and take precedence in resource 
consumption. That being said, the cluster still requires enough resources to 
support both the composite aggregation search and the indexing of its results. 

TIP: If the cluster experiences unsuitable performance degradation due to the
{transform}, stop the {transform} and refer to <<transform-performance>>.


[discrete]
[[ml-transform-checkpoint-heuristics]]
== Change detection heuristics

When the {transform} runs in continuous mode, it updates the documents in the
destination index as new data comes in. The {transform} uses a set of heuristics
called change detection to update the destination index with fewer operations.

In this example, the data is grouped by host names. Change detection detects 
which host names have changed,  for example, host `A`, `C` and `G` and only 
updates documents with those hosts but does not update documents that store 
information about host `B`, `D`, or any other host that are not changed.

Another heuristic can be applied for time buckets when a `date_histogram` is 
used to group by time buckets. Change detection detects which time buckets have 
changed and only update those.


[discrete]
[[ml-transform-checkpoint-errors]]
== Error handling

Failures in {transforms} tend to be related to searching or indexing.
To increase the resiliency of {transforms}, the cursor positions of
the aggregated search and the changed entities search are tracked in memory and
persisted periodically.

Checkpoint failures can be categorized as follows:

* Temporary failures: The checkpoint is retried. If 10 consecutive failures
occur, the {transform} has a failed status. For example, this situation might 
occur when there are shard failures and queries return only partial results.
* Irrecoverable failures: The {transform} immediately fails. For example, this 
situation occurs when the source index is not found.
* Adjustment failures: The {transform} retries with adjusted settings. For 
example, if a parent circuit breaker memory errors occur during the composite 
aggregation, the {transform} receives partial results. The aggregated search is 
retried with a smaller number of buckets. This retry is performed at the 
interval defined in the `frequency` property for the {transform}. If the search 
is retried to the point where it reaches a minimal number of buckets, an 
irrecoverable failure occurs.

If the node running the {transforms} fails, the {transform} restarts from the 
most recent persisted cursor position. This recovery process might repeat some 
of the work the {transform} had already done, but it ensures data consistency.
