Context-level policies can be created to manage EC (erasure coding) across the cluster. Swarm ships with cluster-level EC encoding unspecified. Unless EC is specified at the object level, every object is fully replicated, regardless of size. 

Set custom EC policies for the encoding (k:p) and the object size minimums applicable to specific domains and buckets (contexts). Context-level policies allow building a wide range of erasure-coding controls to meet business needs:

• Disable EC on a bucket, forcing it to perform full replication on every object

• Quadruple the object size triggers erasure coding in a special-use bucket

• Set different EC encodings for named versus unnamed objects in a specific domain

• Lock the domain policy as "unspecified" to cancel any policies bucket owners may have set, forcing them to accept the current cluster policy

• Lock the cluster policy as "disabled" to stop all erasure coding, voiding any query argument or lifepoint encodings passed in with the request

It is necessary to understand how Swarm chooses among different encoding (k:p) to design EC policies because Swarm allows specifying them at multiple levels simultaneously. Encoding specified at the object level wins over any held by the context; the context policy evaluation can disable the encoding requested at the object level:

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An individual object can specify EC encoding in the lifepoint header, as reps=k:p. Add a query argument, as encoding=k:p to override the lifepoint on an SCSP POST or PUT method. Adding the optional erasurecoded argument overrides the active policy.ecMinStreamSize requirement, but it cannot override an EC policy evaluating to "Disabled".

See https://perifery.atlassian.net/wiki/spaces/public/pages/2443821972 for setting lifepoints with erasure coding.

See https://perifery.atlassian.net/wiki/spaces/public/pages/2443821292  for using erasure coding query arguments.

Options for Specifying EC Encoding

A different EC encoding policy can be defined at the cluster and each context (domain and bucket) level, using this 2-part setting:

EC encoding is subject to policy resolution on each write request, evaluating the object-level encoding with the context. For contexts, Swarm uses the policy at the given level unless a level above it is anchored. Swarm uses the anchored encoding.

Anchoring allows enforcing these two situations:

Disable erasure-coding cluster-wide

Require objects to specify encoding, regardless of context:

How Swarm Resolves EC Policies

Swarm begins EC policy evaluation by eliminating contexts (domain and bucket) not applicable, for one of two reasons:

• Type of object 

• "anchored" override 

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Swarm (1) selects the anchored policy if it exists, or else (2) checks the relevant contexts from the bottom up and selects the first policy it finds: 

Example EC Policy Resolution

For example, consider a set of conflicting context-level policies such as this:

The EC policy evaluation resolves differently depending on the type of object being written:

How to View and Set EC Encoding Policy

EC Encoding is one of several content protection features in Swarm, all are controlled by way of policies. Swarm evaluates policies for each object based on cluster and context values:

• Policy-related settings for the cluster (required) 

• Policy-related headers on domain and bucket objects (optional)

For setting policies at the cluster level, see .

Creating a policy involves changing one or both EC requirements for the given scope:

ecEncoding - Whether to allow/prevent/change the EC encoding 

ecMinStreamSize* - Whether to change the object size triggering erasure-coding. 

Set encoding policies with the appropriate naming and method, starting with the cluster, once decided on the encoding policy for each level:

This new domain has a separate required policy for named objects (which bucket owners are not able to override), but unnamed objects follow the current cluster-level policy:

Writing a domain policy

This new bucket disables erasure-coding, but this policy is canceled by any policies "anchored" above it at the domain or cluster level:

Writing a bucket policy