`MibNode.setValue()` is no longer defined for readonly OIDs

[thom-nic]

We have dynamic handlers for many read-only OIDs. For example:

const SYS_OID = '1.3.6.1.2.1.1';

class SystemProvider extends MibProvider {
  constructor(mib) {
    super(mib);
    this._handlers = [
      {
        name: 'sysUpTime',
        oid: `${SYS_OID}.3`,
        type: MibProviderType.Scalar,
        scalarType: ObjectType.TimeTicks,
        maxAccess: MaxAccess['read-only'],
        value: 0,
        handler: (mibRequest) => {
          mibRequest.instanceNode.setValue(ObjectType.TimeTicks, Math.min(Math.round(os.uptime() * 100), 2**32 - 1));
          mibRequest.done();
        },
      },
      ....

I see in v3.25, MibNode.setValue() was removed: 3.24.0...3.25.0#diff-e727e4bdf3657fd1d798edcd6b099d6e092f8573cba266154583a746bba0f346L3986

Is the correct migration path to do something like this instead?

        handler: (mibRequest) => {
          const { instanceNode } = mibRequest;
          const constraints = instanceNode.getConstraintsFromProvider();
          const value = Math.min(Math.round(os.uptime() * 100), 2**32 - 1);
          const castValue = ObjectTypeUtil.castSetValue(ObjectType.TimeTicks, value, constraints);
          if ( ! instanceNode.validateValue(ObjectType.TimeTicks, castValue) {
            // throw error?
          }
          instanceNode.value = castValue;
          mibRequest.done();
        },

[markabrahams]

Hi @thom-nic - thanks for the report. Yes, that change in 3.25 wasn't well documented, sorry.

The short answer is that you don't need to reproduce the cast, validate, assign sequence in your handler. For a dynamic read-only scalar like sysUpTime, where the value is computed by the server rather than supplied by an SNMP peer, the canonical migration is just to assign the new value to instanceNode.value before calling done():

handler: (mibRequest) => {
  mibRequest.instanceNode.value = Math.min(Math.round(os.uptime() * 100), 2**32 - 1);
  mibRequest.done();
}

The response varbind is read from mibRequest.instanceNode.value at the moment done() is called, so that's all that's required. If you want validated assignment, the equivalent using the public MIB API is:

handler: (mibRequest) => {
  mib.setScalarValue('sysUpTime', Math.min(Math.round(os.uptime() * 100), 2**32 - 1));
  mibRequest.done();
}

This goes through ObjectTypeUtil.castSetValue and constraint checking internally and throws on a bad value.

I've updated the README to document this - the scalar provider example now shows the read-only dynamic pattern, and the handler reference paragraph spells out the Get vs. Set contract and points at some validation helpers for the rare cases where a handler genuinely needs to validate an external value itself.