Managing tasks

Important: All Cloudant documentation has moved to the IBM Bluemix platform. You can find the new content here, and the ‘Managing tasks’ topic in particular here.

Content on this page will no longer be updated (Jan 31st, 2017).

Creating new indexes over lots of data or replicating a large database can take quite a while. So how can you see whether your tasks are making progress, or if they have completed? The _active_tasks endpoint provides information about all ongoing tasks. However, if you start a lot of tasks, some of them might be scheduled to run later and do not show up under _active_tasks until they have been started.

This guide tells you how to use the _active_tasks endpoint to monitor long-running tasks. The curl command is used to access the endpoint. The jq command-line JSON processor is used to process the JSON response.

Since this is a task-focused tutorial, it covers only what is essential to accomplish this task. Please refer to the API reference for a complete guide to the available options.

curl and jq basics

curl '' | jq '.'

To get all active tasks and format the output nicely, call your account using curl, and pipe the output to jq.

jq lets you filter a list of documents by their field values. This makes it easier to get all replication documents, or the details of just one particular view indexing task. The API reference has more information about the options.

Monitoring view builds and search indexes

curl -s '' | jq '.[] | select(.type=="indexer")'
curl -s '' | jq '.[] | select(.type=="search_indexer")'
  "total_changes": 6435,
  "started_on": 1371118332,
  "user": "username",
  "updated_on": 1371118334,
  "type": "indexer",
  "node": "",
  "pid": "<0.16366.6103>",
  "changes_done": 364,
  "database": "shards/40000000-7fffffff/username/database",
  "design_document": "_design/ngrams"

View indexes are rebuilt when a design document is updated. An update to any one of the views causes all the views in the document to be rebuilt.

Search indexes are rebuilt only when their corresponding index function is changed. For each search index that is being built and for each design document with views that are changed, a new task is created for each replica of each shard in a cluster. For example, if there are 24 shards, with three replicas each, and you update two search indexes, then 24 x 3 x 2 = 144 tasks are run.

To find all the view indexing tasks, pipe the curl output to jq, and let it filter the documents in the array by their type field. A corresponding command works for search indexing tasks.

In each case, the results of searching for a list of indexing tasks is a list of JSON objects: one for each of the active tasks found.

Estimating the time to complete a task

curl ... | jq '.[] | select(.type=="search_indexer") | .changes_done'

To estimate the time needed until the indexing task is complete, monitor the number of changes_done and compare this value to total_changes. For instance, if changes_done increases by 250 per second and total_changes is 1,000,000, the task is expected to take 4,000 seconds, or about 66 minutes, to complete.

Monitoring replication

curl -s '' | jq '.[] | select(.type=="replication")'
curl ... | jq '.[] | select(.doc_id=="ID")'
curl ... | jq '.[] | select(.replication_id=="ID")'
  "started_on": 1371094220,
  "source_seq": "62960-sakdjflksdfjsdlkafjalskdfjlsakfjlasdkjksald",
  "source": "",
  "revisions_checked": 12,
  "continuous": true,
  "doc_id": null,
  "doc_write_failures": 0,
  "docs_read": 12,
  "target": "",
  "type": "replication",
  "updated_on": 1371118477,
  "user": "username",
  "checkpointed_source_seq": "61764-dskfjalsfjsalkfjssadjfhasdfkjhsdkfhsdkf",
  "changes_pending": 1196,
  "pid": "<0.9955.4120>",
  "node": "",
  "docs_written": 12,
  "missing_revisions_found": 12,
  "replication_id": "asfksdlfkjsadkfjsdalkfjas+continuous+create_target"

To find all replication tasks, pipe the curl output to jq, and filter the documents in the array by their type field.

To make it easier to select the information about a replication process from the list of active tasks, start the replication process by creating a document in the _replicator database, and set its _id field to a known value.


Is a task stuck?

For a one-off, non-continuous replication, where the source database is not updated significantly during the replication, the changes_pending value tells you how many documents remain to be processed. This means that the changes_pending value is good indicator of when the replication is likely to be finished.

For a continuous replication, you are more interested in how the number of documents processed changes over time, and whether the changes_pending value increases. If changes_pending increases, but revisions_checked stays constant for a while, the replication is probably stalled. If changes_pending increases, and revisions_checked also increases, this might indicate that the replication cannot keep up with the volume of data added to, or updated in, the database.

What to do about a stuck task?

To resolve a stalled replication, you might have to cancel the replication process and start it again.

If that does not help, the replication might be stalled because the user accessing the source or target databases does not have write permissions.

If you created the replication process by creating a document in the _replicator database, you can also check the status of the replication there.