Q_AND_A.md

DWN Q&A

Message ID (messageCid), Record ID (recordId) and Context ID (contextId)

• Why can't/don't we use the message ID (messageCid) of the initial RecordsWrite as the record ID?

  (Last updated: 2023/05/16)

  Because the initial RecordsWrite references the record ID in multiple places, so this would be a "chicken-and-egg" problem:

  1. recordId property contains the record ID
  2. If the initial RecordsWrite is for a root record in a DWN protocol, the record ID is the value of the contextId property
  3. If the initial RecordsWrite is encrypted, the record ID is used in the final segment of the key derivation path

  Because of the above constraints, we had to introduce a different algorithm (a.k.a as entryId generation) to deterministically compute the recordId.

  See question why is recordId needed in the RecordsWrite for more info.

• Why is recordId required in a RecordsWrite?

  (Last updated: 2024/02/06)

  This question should be further split into two:

  1. Why is recordId required in an initial RecordsWrite (create)?
  2. Why is recordId required in a subsequent RecordsWrite (update)?

  The latter question is much easier to answer: an update needs to reference the record that it is updating.

  The answer to the first-part question is more complicated: recordId technically is not needed in an initial RecordsWrite, but we chose to include it for data model consistency with subsequent RecordsWrite, such that we can simply return the latest message of a record as the response to RecordsRead and RecordsQuery without needing to re-inject/rehydrate recordId into any initial RecordsWrite. It is also the same reason why contextId is required for the initial RecordsWrite of a protocol-authorized record.

• Why is recordId and contextId outside the descriptor.

  • Because of the chicken-and-egg problem: recordId computation requires the descriptor as the input, so we cannot have recordId itself as part of the descriptor. contextId is similar in the sense that a record's contextId contains its own recordId, so it also cannot be inside the descriptor.

  (Last updated: 2024/02/07)

• Why do we require contextId for protocol-based RecordsWrite? Can't it be inferred from parentId and protocolPath?

  Yes, it can be inferred. But it is required for the same reason why recordId is required: for both implementation and developer convenience.

  For example: for decryption, one would need to know the contextId to derive the decryption key at the right context, without this information readily available, the client would need to compute this value by walking up the ancestral chain themselves.

  An alternative viable approach is to still not require it in RecordsWrite message and compute it internally, and return a constructed contextId as additional metadata along side of the RecordsWrite message when handling a query. This could incur cognitive load on the developers because they will likely need to pass the contextId in addition to the RecordsWrite message around instead of just passing RecordsWrite message. This would also mean we need to store this constructed contextId in the store as metadata (not just as index) so that we can return it as part of the a query (e.g. looking up the contextId of the parent). While this is a bigger change, open to feedback if this is indeed the preferred approach.

  (Last update: 2024/02/07)

• Why does the contextId include the recordId of the record itself? Couldn't we adopt an alternative approach where the contextId is a path that ends at a record's parent?

  Yes, we could opt to exclude the recordId of the record from the contextId of the record itself. However, this would complicate the process of querying for all records of a given context when the root record itself needs to be included. For instance, if we have a root "Thread" context record and we want to retrieve all the records of this Thread, including the root Thread record, the absence of a contextId containing its own recordId would necessitate a separate or more complex query to fetch the Thread record.

  (Last update: 2024/02/07)

Encryption

• Why is publicKeyId required in KeyEncryptionInput?

  (Last updated: 2023/05/19)

  It is required because the ID of the public key (more precisely the ID of the asymmetric key pair) used to encrypt the symmetric key will need to be embedded as metadata (named rootKeyId), so that when a derived private key (which also contains the key ID) is given to the SDK to decrypt an encrypted record, the SDK is able to select the correct encrypted key for decryption. This is useful because if there are multiple encrypted keys attached to the record, the correct encrypted key will be selected immediately without the code needing to trial-and-error on every key until a correct key is found.

  Even if there is only one encrypted key attached to the encrypted record, there is no guarantee that the private key given is the correct corresponding private key, so it is still important to have the key ID so that the code can immediately reject the given private key if the ID does not match. There are a number of cases why a key ID mismatch can occur:

  1. The DWN owner might have published multiple encryption keys, and a wrong encryption key is chosen.
  2. The key used to encrypt the record might not be the DWN owner's key at all. For instance, a sender's encryption key is used instead.

• Instead of introducing yet another property publicKeyId in KeyEncryptionInput, why do we not just use the kid property in the public JWK?

  (Last updated: 2023/05/19)

  This is because:

  1. kid is an optional property of a JWK, there is no guarantee that the public JWK will contain it.
  2. In the future public key may not always be given in JWK format. A key in raw bytes does not contain metadata such as key ID.

Pagination

• Why is messageCid mandated as the cursor for pagination?

  (Last updated: 2023/09/12)

  The requirement for using messageCid as the cursor for pagination aims to ensure compatibility irrespective of DWN store implementations. The goal is for a query with the same cursor to yield identical results, regardless of which DWN is handling the query. This is useful because, if a DWN becomes unavailable after delivering a page of messages, the caller can switch to another DWN and resume fetching subsequent pages without interruption.

Protocol

• Can a record that is not protocol-authorized have protocol property in its descriptor?

  (Last updated: 2023/05/23)

  No.

• When making RecordsQuery or RecordsSubscribe by invoking a protocol role, why is protocolPath a required filter property? This means that one cannot filter records under a protocol or contextId irrespective of the protocolPath, thus is forced to make multiple queries (ie. one per protocolPath).

  (Last update: 2023/11/03)

  This design choice is primarily driven by performance considerations. If we were to make protocolPath optional, and it is not specified, we would need to search records across protocol paths. Since protocol rules (protocol rule set) are defined at the per protocol path level, this means we would need to parse the protocol rules for every protocol path in the protocol definition to determine which protocol path the invoked role has access to. Then, we would need to make a database query for each qualified protocol path, which could be quite costly. This is not to say that we should never consider it, but this is the current design choice.

• What is the difference between write and co-update actions?

  (Last update: 2024/03/04)

  • write - allows a DID to create and update the record they have created
  • co-update - allows a DID to update a record, regardless of the initial author

• What is the difference between the terms "global role" and "context role"?

  (Last update: 2024/02/16)

  The structure and usage of "global roles" and "context roles" are identical. The distinction lies in their placement within the protocol hierarchy: a "global role" is defined as a root level record without a parent record, whereas a "context role" has a parent record thus making it "contextual". This means that a protocol rule set can have access to all global role records/assignments, hence the "global" designation; conversely, access to context role records/assignment is restricted to the specific context relevant to the message being authorized.

  Subscriptions

• What happens to a subscription which is listening to events, but is no longer authorized due to revocation of a grant or role?

  (Last update: 2024/01/23)

  Currently if a subscription is no longer authorized but it is still active, the subscriber will still receive updates until they close the subscription themselves. If they try to re-subscribe after that, it will be rejected with a 401.

  This will be addressed in a future upgrade and we've created an issue to track it. decentralized-identity#668 - last updated (2024/01/22)

• Why are we not notifying deletes in a subscription that uses mutable property as a filter (e.g. published, dataFormat).

  (Last update: 2024/02/29)

  We are happy to revisit but the current behavior is due to the following arguments:

  • Philosophical argument: the subscription filter is subscribing to events/messages that matches the filter, not changes to an earlier state.
  • Practical argument: this requires more thought and coding and is a lower priority until we have real-world usage and feedback.

  Data Store

• Is it possible to implement the Data Store interface purely using a blob/binary data storage service such as Amazon S3, Azure Blob Storage, or Google Cloud Storage?

  (Last update: 2024/01/30)

  The short answer is: yes.

  The long answer, with context:

  Keys to objects are generally immutable once the object is created by all three vendors. Amazon S3 and Google Cloud Storage do not have built-in mechanisms to search by their metadata/tags on the server side, even though they support the modification of metadata/tags. Only Azure Blob Storage allows both search and modification of metadata for a written object. Amazon S3 and Google Cloud Storage also have limited support for "partitions": a finite limit in S3 or a limited rate of 1 per 2 seconds, while Azure Blob Storage fully supports partitions.

  All of the above means that an implementation using Azure Blob Storage could be the most "clean". However, it should still be straightforward to implement the Data Store using Amazon S3 or Google Cloud Storage by using recordId + dataCid as the object key.

  Implementers have the liberty to introduce advanced features such as reference counting to avoid storing the same data twice, but this is not a requirement.