DELETE
This article provides an overview and how-to instructions for deleting documents using the EVICT DQL operation.
For complete DQL syntax, see Ditto Query Language (DQL) > EVICT.
The EVICT operation functions based on a condition, allowing updates to multiple documents simultaneously.
For example, the following snippet, once executed, purges all blue cars stored in the local Ditto store.
Once removed, you can reference the evicted document using the mutatedDocumentIDs method on the result.
To clear documents with active subscriptions, you must first cancel the relevant subscription before calling the EVICT method.
You must manage subscriptions and evictions carefully.
If subscriptions are not properly managed prior to executing evictions, you may inadvertently disrupt the intended state, resulting in inconsistencies and unexpected behavior. For instance, the eviction process failing and the document persisting in the local Ditto store.
For example, if you have an active subscription for fetching 'blue' cars and you subsequently evict the document with the ID '123456' that matches the replication query, connected peers reinstate it in your local Ditto store. In other words, the document does not clear and remains available in the local Ditto store.
In addition, take a balanced approach when using the Subscribe and Evict methods. Consider the advantages and drawbacks of each method and use them as appropriate for specific needs and requirements.
Key considerations for using Subscription and Eviction methods include:
- Use Subscribe to sync more data across connected peers in the mesh; however, be mindful of potential increased network usage, which may degrade sync performance.
- Use Evict to manage local storage capacity and improve performance by routinely purging data stored locally.
Attachment data stored within the Small Peer Ditto store is automatically garbage collected on a 10-minute cadence when no longer referenced.
Attachments currently can only be deleted by way of garbage collection.
Unlike documents, attachments cannot be explicitly deleted on their own. Instead, you modify the document containing the attachment token referencing it.
The following table provides an overview of the various ways you can indirectly delete attachments:
Approach | Description |
|---|---|
UPDATE | Update the document to remove the associated attachment token. |
EVICT | Delete the entire document, including the associated attachment token, from the Ditto store. |
The storage mechanism Small Peers use to store data, including blob data, depends on the platform:
- If running in the browser or a server-based system, data is stored in its Random Access Memory (RAM).
- If running on a mobile device like an iPhone, data is stored on its local filesystem.
To add a soft-delete pattern, set the isArchived field value to true:
To query to monitor documents that are NOT archived, establish a live query where isArchived is set to false, and then construct your live query callback.
For example, the following code demonstrates searching for documents that are unarchived:
To remove the flag and reactivate the document, set the isArchived field to false:
In peer-to-peer system design, there are technical tradeoffs between the amount of data synced across peers and the timeliness of access to synced data:
- The greater the amount of data synced, the more timely offline read access becomes. That is, database resilience in offline scenarios increases when more documents are being synced across distributed peers.
- The fewer the number of documents replicated, the less the likelihood that peer devices run out of disk space and experience memory leaks, and the performance of the peer-to-peer mesh network that interconnects them degrades.
For considerations on using the Evict and Subscribe methods, see Timing Subscriptions and Evictions.
For advanced concepts related to design tradeoffs in distributed system architecture, see Cloud-Optional Design.