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## Migration from H2 Caches
This module provides a REST API to help converting existing cache from H2 to
chronicle-map, which requires the `Administrate Caches` or `Administrate Server`
capabilities to be executed.
The migration must be executed _before_ switching to use chronicle-map, while
Gerrit cache is still backed by H2.
The migration can be run online without any disruption of the Gerrit server.
However note that since the migration perform many, sequential reads from the H2
database, it will participate to the overall database load, so you should test
accordingly.
The migration would do the following:
1. scan all existing cache key-value pairs
2. calculate the parameters for the new cache, if not already defined in gerrit.config.
3. create the new cache
4. read all existing key-value pairs and insert them into the new cache-chroniclemap files
> **NOTE**: The existing cache parameters are kept in `gerrit.config` only when they are all
> defined (avgKeySize, avgValueSize, maxEntries and maxBloatFactor), otherwise the
> migration process will recalculate them and create the new cache based on the new
> values.
The following caches will be migrated (if they exist and contain any data):
* accounts
* change_kind
* change_notes
* conflicts
* diff
* diff_intraline
* diff_summary
* git_tags
* mergeability
* oauth_token
* persisted_projects
* pure_revert
* web_sessions
The migration should be performed as follows:
* Copy `cache-chroniclemap.jar` file in the `plugins/` directory.
* Wait for the pluginLoader to load the new plugin. You will see an entry in
the `error_log`:
```
INFO com.google.gerrit.server.plugins.PluginLoader : Loaded plugin cache-chroniclemap
```
* You can now run the migration
```bash
curl -v -XPUT -u <admin> '<gerrit>/a/plugins/cache-chroniclemap/migrate?[size-multiplier=FACTOR]&[bax-bloat-factor=MULTIPLIER]'
```
This might require some time, depending on the size of the H2 caches and it will
terminate with the output of the configuration that should be places in
`etc/gerrit.config`in order to leverage the newly created caches correctly.
Output example:
```
[cache "diff"]
maxEntries = 216
avgKeySize = 188
avgValueSize = 796
maxBloatFactor = 1
[cache "persisted_projects"]
maxEntries = 3
avgKeySize = 80
avgValueSize = 4087
maxBloatFactor = 1
[cache "diff_summary"]
maxEntries = 216
avgKeySize = 192
avgValueSize = 254
maxBloatFactor = 1
[cache "accounts"]
maxEntries = 2
avgKeySize = 52
avgValueSize = 194
maxBloatFactor = 1
[cache "mergeability"]
maxEntries = 2444
avgKeySize = 150
avgValueSize = 20
maxBloatFactor = 1
[cache "web_sessions"]
maxEntries = 94852
avgKeySize = 68
avgValueSize = 382
maxBloatFactor = 1
```
Optionally the REST endpoint can receive the following additional arguments:
* max-bloat-factor=FACTOR
maximum number of times chronicle-map cache is allowed to grow in size.
*default:3*
* size-multiplier=MULTIPLIER
Multiplicative factor for the number of entries allowed in chronicle-map.
*default:3*