Purging

Only available in 3.7.0 and above

Purging is a tool that allows you to increase performance and available disk space for offline users (eg CHWs) by removing unneeded documents from their device.

As users continually generate new reports their performance may naturally degrade as a result. You can use purging to remove older documents that are no longer relevant from their devices. Purging only removes documents from user’s devices: these reports are still available for online analytics and impact metrics.

Purging is disabled by default, and is enabled if a purge function is specified in app_settings.json, along with a run schedule.

The following example would purge all reports that were created more than a year ago:

{
  "//": "other app_settings settings",
  "purge": {
    "fn": "function(userCtx, contact, reports, messages) { const old = Date.now() - (1000 * 60 * 60 * 24 * 365); return reports.filter(r => r.reported_date < old).map(r => r._id);}",
    "text_expression": "at 12 am on Sunday"
  }
}

Purging is both very powerful and also very dangerous. Read the rest of this document carefully to make sure you completely understand how to purge and the ramifications of doing so, before using purging in your project.

Server-side

Purging runs on the server on a configurable schedule.

It will iterate over all users to generate a list of unique roles groups that represent every user. Each group will have their purged docs saved in an individual database.

Then, it will iterate over all existent contacts, collecting all reports about that contact along with all sms messages that the contact has sent or received. This is similar to the scoping you may have encountered when configuring tasks and targets.

The configured purge function runs over all combinations of purge scope (contact + reports + messages) and user context (unique list of roles) to determine which docs should be purged.

The resulting list of docs to be purged is compared to the existent purged docs so that only the differences are saved (old purges are reverted and new purges are added).

A document is considered purged for a user if a document with the same id, prefixed by purge, exists in the corresponding purge database.

The following user:

{
  "name": "org.couchdb.user:<your user>",
  "roles": [
    "district_admin",
    "supervisor"
  ]
}

would get their purges from a medic-purged-role-<role_hash> where role_hash is an md5 hash of the user’s roles.

When users sync (includes initial sync), they will only download documents that are not purged for their roles.

Client-side

Periodically a user’s device calls an API endpoint that returns batches of doc ids that have been purged since last time the same device checked. This is done in the background so the user is not disrupted from their work.

The system is similar to CouchDB replication, in the sense that a checkpointer document is saved in the corresponding server-side database, that stores the last_seq that the device has downloaded and is used to get the next batch of ids. Purging many documents while the application is running is an expensive and potentially disruptive activity, so instead the ids are stored until the next time the app is started.

Then, when the user launches the app again during the bootstrap phase the app checks if there are any documents to purge, and if so deletes them all before starting up as usual. Doing the deletion prior to startup is relatively fast and designed to disrupt the user as little as possible.

Configuration

To enable purging, write your purge configuration to purge.js in your project root:

module.exports = {
  text_expression: 'at 9 am on Sunday',
  run_every_days: 7,
  cron: '0 1 * * SUN',
  fn: function(userCtx, contact, reports, messages) {
    const old = Date.now() - (1000 * 60 * 60 * 24 * 365);
    const oldMessages = Date.now() - (1000 * 60 * 60 * 24 * 90);

    const reportsToPurge = reports
      .filter(r => r.reported_date < old)
      .map(r => r._id);
    const messagesToPurge = messages
      .filter(m => m.reported_date < oldMessages)
      .map(m => m._id);

    return [...reportsToPurge, ...messagesToPurge];
  }
};

Purge configuration

As shown above, you should be exporting a property fn defining a self contained function: it should have no outside dependencies - like used variables, required modules or call outside functions.

This function takes four parameters:

• userCtx, an object with the user’s roles as fields. For more information read the documentation for the User Context Object.
• contact, the contact document of a patient or other contact who has reports about them.
• reports, an array of all reports for that subject that are present on the server.
• messages, an array of sms messages that the contact has sent or received

And should return an array of _id values for docs you would like to be purged (or undefined / nothing if you don’t wish to purge anything). `Only ids of docs that were passed to the function are valid for purging: you are not allowed to purge other documents.

In the cases of reports that do not have subjects or their subjects are not found, the purge function will receive an empty object as contact. In the cases of reports about deleted contacts, the purge function will receive a { _deleted: true } object as the contact.

As of 3.9.0, task documents that are in a terminal state (Cancelled, Completed, Failed) are purged if their end_date is more than 60 days ago (relative to server date). As of 3.9.0, target documents are purged if their reporting period is more than 6 months ago (relative to server date). Purging task and target documents happens automatically on every purge run. The intervals and required states are not configurable.

Schedule configuration

You must set a schedule for purging to run server-side. Depending on the size of the database and server capacity, purging could be a lengthy and resource intensive operation, so it is recommended you run purge on a schedule that your server can sustain (for example at nighttime in the weekends).

You can also change the frequency of local purge downloads (default being every 7 days).

Example of purge configured in your app_settings:

  "//": "other app_settings settings",
  "purge": {
    "fn": "function(userCtx, contact, reports, messages) { return  []; }",
    "cron": "0 1 * * SUN",
    "text_expression": "at 1:00 am on Sun",
    "run_every_days": 5
  }

Considerations

Purged documents server-side

Purging is run as a scheduled task in Sentinel.

Purging does not touch documents in the medic database, everything is done in separate purge databases(medic-purged-roles-<roles-hash>).

The purge databases names contain an md5 of the JSON representation of a list of unique roles. They also contain a _local/info doc where the roles are listed in clear text.

As of 3.14.0, contacts that have more than 20,000 associated reports + messages will be skipped, and none of their associated reports and messages will be purged. A single contact that has more than 20,000 associated records most likely points to a configuration issue. Skipped contacts’ ids are reported both in logs and in purgelog files (see below).

A purgelog document is saved in the medic-sentinel database after every purge. The purgelog has a meaningful id: purgelog:<timestamp>, where timestamp represents the moment when purging was completed. As of 3.14.0, every time purging fails to complete due to an error, a purgelog:error document is saved in the medic-sentinel database. The ids of these documents are similarly meaningful: purgelog:error:<timestamp>.

purgelog docs have the following properties:

You can retrieve a list of all your purge logs, descending from newest to oldest, with this request:

https(s)://<host>/medic-sentinel/_all_docs?end_key="purgelog:"&start_key="purgelog:\ufff0"&descending=true

You can retrieve a list of all your purge logs with errors, descending from newest to oldest, with this request:

https(s)://<host>/medic-sentinel/_all_docs?end_key="purgelog:error:"&start_key="purgelog:error:\ufff0"&descending=true

Purging is reversible. If you update your purge function, when running purge the old invalid purges will be deleted. This does not mean that devices will automatically re-download documents that become unpurged. In order for the user to re-download a previously purged document, the document either needs to be updated in the medic database on the server or the user has to download all data again.

Running purge will not remove old purge databases, even if they don’t correspond to any existent users. Their removal is a manual process.

Purge does not run when adding new roles or adding said new roles to users. It also does not run when an existent user is updated to have a new unique list of roles (one that purge has not run over yet). This means that roles need to be planned carefully in order to take advantage of serverside purge. If purge has not run for the user’s list of roles at the moment of initial replication, the user will download all documents - only to be purged later.

Purged documents client-side

The key thing to keep in mind while purging is that documents that you purge are deleted on user’s device. This sounds obvious, but it’s important to understand how this affects the running of the application:

• Any rules you have written that presume that the document exists may break. For example, if the document completes a task, purging it will reopen that task, unless you also purge the document that created the task in the first place (while making sure that purging that report doesn’t break more things!)
• Similarly targets won’t be able to use the report to generate values, so counts may go down or become inaccurate
• Additionally, the contact summary will also lose out on being able to use that report
• Changing the user’s roles list (adding/removing roles) will cause the user to download all purged docs ids from the purge database corresponding to their new roles list.

More subtly, you may also confuse your users!

If you purge documents too quickly, they may get confused as to whether they created the report or not, and may create it again, causing data problems. Users are not told that purging is occurring in a very obvious way: the expectation is that purging will naturally occur as documents become irrelevant, and so users should never really notice.

Users may search for their own documents, and use data from them in novel ways you may not anticipate. It’s important to work with your users to ensure documents are only removed once there are no uses for them.

It is key then, that you test your purge rules thoroughly!

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