Merge pull request #1554 from hyperledger/docs_v1.3.1_awr

Additional info and clarification in listener docs

Author: awrichar

doc-site/docs/reference/types/_includes/contractlistener_description.md

@@ -7,51 +7,95 @@ a walk-through of how to set up listeners for the events from your smart contrac
 
 See below for a deep dive into the format of contract listeners and important concepts to understand when managing them.
 
-### Multiple filters
+### Event filters
 
-From v1.3.1 onwards, a contract listener can be created with multiple filters under a single topic when supported by the connector.
+#### Multiple filters
 
-Before this change, each contract listener would only support listening to one specific event from an interface previously defined. Each listener would be comprised of:
+From v1.3.1 onwards, a contract listener can be created with multiple filters under a single topic, when supported by the connector. Each filter contains:
 
 - a reference to a specific blockchain event to listen for
 - (optional) a specific location/address to listen from
-- a connector-specific signature (generated from the event), which allows you to easily identify and search for the contact listener for an event
-- a `topic` which determines the ordered stream that these events are part of
+- a connector-specific signature (generated from the event and the location)
 
-This format is still supported by the API. However, it may not fully guarantee accurate ordering of events coming from multiple listeners, even if they share the same `topic` (such as during cases of blockchain catch-up, or when one listener is created much later than the others).
+In addition to this list of multiple filters, the listener specifies a single `topic` to identify the stream of events.
 
-From v1.3.1, you can supply multiple filters on a single listener. Each filter contains:
+Creating a single listener that listens for multiple events will allow for the easiest management of listeners, and for strong ordering of the events that they process.
+
+#### Single filter
+
+Before v1.3.1, each contract listener would only support listening to one specific event from a contract interface. Each listener would be comprised of:
 
 - a reference to a specific blockchain event to listen for
 - (optional) a specific location/address to listen from
-- a connector-specific signature (generated from the event and the location)
+- a connector-specific signature (generated from the event), which allows you to easily identify and search for the contact listener for an event
+- a `topic` which determines the ordered stream that these events are part of
+
+For backwards compatibility, this format is still supported by the API.
+
+### Signature strings
+
+#### String format
+
+Each filter is identified by a generated `signature` that matches a single event, and each contract listener is identified by a `signature` computed from its filters.
+
+Ethereum provides a string standard for event signatures, of the form `EventName(uint256,bytes)`. Prior to v1.3.1, the signature of each Ethereum contract listener would exactly follow this Ethereum format.
+
+As of v1.3.1, Ethereum signature strings have been changed, because this format does not fully describe the event - particularly because each top-level parameter can in the ABI definition be marked as `indexed`. For example, while the following two Solidity events have the same signature, they are serialized differently due to the different placement of `indexed` parameters, and thus a listener must define both individually to be able to process them:
+
+- ERC-20 `Transfer`
+
+  ```solidity
+  event Transfer(address indexed _from, address indexed _to, uint256 _value)
+  ```
+
+- ERC-721 `Transfer`
+
+  ```solidity
+  event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId);
+  ```
+
+The two above are now expressed in the following manner by the Ethereum blockchain connector:
+
+```solidity
+Transfer(address,address,uint256) [i=0,1]
+Transfer(address,address,uint256) [i=0,1,2]
+```
+
+The `[i=]` listing at the end of the signature indicates the position of all parameters that are marked as `indexed`.
+
+Building on the blockchain-specific signature format for each event, FireFly will then compute the final signature for each filter and each contract listener as follows:
 
-In addition to this list of multiple filters, the listener specifies a single `topic` to identify the stream of events. This new feature will allow better management of contract listeners and strong ordering of all of the events your application cares about.
+- Each filter signature is a combination of the location and the specific connector event signature, such as `0xa5ea5d0a6b2eaf194716f0cc73981939dca26da1:Changed(address,uint256) [i=0]`
+- Each contract listener signature is a concatenation of all the filter signatures, separated by `;`
 
-Note: For backwards compatibility, the response from the API will populate top-level `event` and `location` fields with the contents of the first event filter in the array.
+#### Duplicate filters
 
-### Duplicate filters
+FireFly restricts the creation of a contract listener containing duplicate filters.
 
-FireFly will restrict the creation of a contract listener with duplicate filters or superset filters. For example, if two filters are listening to the same event, but one has specified a location and the other hasn't, then the latter will be a superset, and already be listening to all the events matching the first filter. Creation of duplicate or superset filters will be blocked.
+This includes the special case where one filter is a superset of another filter, due to a wildcard location.
 
-### Duplicate listeners
+For example, if two filters are listening to the same event, but one has specified a location and the other hasn't, then the latter will be a superset, and already be listening to all the events matching the first filter. Creation of duplicate or superset filters within a single listener will be blocked.
 
-As of v1.3.1, each filter on a listener includes a signature generated from the filter location + event, and the listener concatenates all of these signatures to build the overall contract listener signature. This contract listener signature - containing all the locations and event signatures combined with the listener topic - will guarantee uniqueness of the contract listener. If you tried to create the same listener again, you would receive HTTP 409. This combination can allow a developer to assert that their listener exists, without the risk of creating duplicates.
+#### Duplicate listeners
 
-Note: Prior to v1.3.1, FireFly would detect duplicates simply by requiring a unique combination of signature + topic + location for each listener. When using muliple filters, we cannot rely on the signature of one event and the location of one filter to calculate uniqueness of a contract listener, hence the need for the more sophisticated uniqueness checks described here.
+As noted above, each listener has a generated signature. This signature - containing all the locations and event signatures combined with the listener topic - will guarantee uniqueness of the contract listener. If you tried to create the same listener again, you would receive HTTP 409. This combination can allow a developer to assert that their listener exists, without the risk of creating duplicates.
 
-### Signature enhancements
+**Note:** Prior to v1.3.1, FireFly would detect duplicates simply by requiring a unique combination of signature + topic + location for each listener. The updated behavior for the listener signature is intended to preserve similar functionality, even when dealing with listeners that contain many event filters.
 
-As mentioned above, we have introduced a new format for signatures of contract listener and filter signature:
+### Backwards compatibility
 
-- Each filter signature will be a combination of the location and the specific connector event signature
-- Each contract listener signature will be a concatenation of all the filter signatures
+As noted throughout this document, the behavior of listeners is changed in v1.3.1. However, the following behaviors are retained for backwards-compatibility, to ensure that code written prior to v1.3.1 should continue to function.
 
-Furthermore, because Ethereum ABI does not include a differentiation between indexed and non-indexed fields, we have included a new section in the Ethereum-specific event signature to add the index of the indexed fields. As such, a filter listening to the event `Changed(address indexed from, uint256 value)` at address `0xa5ea5d0a6b2eaf194716f0cc73981939dca26da1` will result in `0xa5ea5d0a6b2eaf194716f0cc73981939dca26da1:Changed(address,uint256) [i=0]` where `[i=0]` specifies that the first field is indexed. If there were more indexed fields, it will be a comma-separated list of the index of those indexed fields such as `[i=0,2,3]` (specifies that fields at index 0, 2, and 3 are indexed fields).
+- The response from all query APIs of `listeners` will continue to populate top-level `event` and `location` fields
+  - The first entry from the `filters` array is duplicated to these fields
+- On input to create a new `listener`, the `event` and `location` fields are still supported
+  - They function identically to supplying a `filters` array with a single entry
+- The `signature` field is preserved at the listener level
+  - The format has been changed as described above
 
-### Formats supported
+### Input formats
 
-As described above, there are two input formats supported by the API for backwards compatibility with previous releases.
+The two input formats supported when creating a contract listener are shown below.
 
 **Muliple Filters**