Causeway Platform Developer
From Erights
Contents |
Instrumenting a Platform to Generate Causeway Trace Logs
Introduction, communicating events loops
- Waterken
- promises
- AmbientTalk
- futures
Getting Started
- See Causeway for user documentation, which includes instructions for launching Causeway from a command line shell.
- See HP Labs Technical Report for a more in-depth discussion of the material presented here. A careful read is not necessary but a quick skim to see what's there is a good idea.
This documentation assumes an understanding of the purchase-order example program.
- Read the source code. It's part of the E distribution and can be found here.
e/src/esrc/scripts/test/causeway/waterken/sourcese/src/esrc/scripts/test/causeway/ambientTalk/sources
Also, we're asking that you write a version of the purchase-order example that runs on your platform, as one of your first test cases. It's best to start with the Ajax-style continuation-passing, using callbacks. This approach is less expressive than promises, but generating the trace log is more straightforward.
- Browse the example programs in Causeway, e.g., Help >> Open Waterken Example (Ajax-style).
Setting Causeway's debug flag enables a Debug view. As events are selected in the viewer, the Debug view shows the corresponding trace record in the log file. This is a very useful option, especially in the beginning, when things aren't quite working.
$ rune -Dcauseway_debug=true causeway.e-swt
(
Note:
When this debug option is set, Causeway's JSON parser uses getTwine(). This is an inefficient algorithm which is not likely to change anytime soon. The trace logs for the example programs are roughly 20K; the poor performance is noticed with logs > 250K.)
Individual tree items (e.g. shown in the message-order view above) represent events and their descriptive labels depend on the information available in the trace record for the event. Causeway labels the tree items according to the following priority.
- The text field string. This field is required for Comment records; it is optional for Sent, SentIf, and Resolved records. (e.g. # Order placed for West Coast Buyers)
- If the trace record has at least one stack entry with a source and span, a single line of source code from the source file specified in the top stack entry. (e.g.
_._(inventory).partInStock(partNo, teller);) - If there's no span, source file name and function name specified in the top stack entry. (e.g.
[buyer, 8] AsyncAnd.run) - If nothing else, a Causeway comment. (e.g.
## unknown sender)
Causeway's Trace Log Format
Causeway supports the trace log format defined by Tyler Close at waterken.sourceforge.net/debug/.
When causality tracing is on the events are logged as follows.
| Event | Record type |
|---|---|
| An eventual send to an object | Sent |
| A message delivery, starting a new turn | Got |
| Registration of a when-block, to execute when a promise resolves | SentIf |
| Returned result from a remote object, for local promise resolution | Returned |
| A state-based event contributes to promise resolution | Progressed |
| A promise resolves | Fulfilled or Rejected |
| Programmer logs a comment | Comment |
Logging Ajax-style Messaging in Waterken
Consider the sequence of events shown below.
- a remote inventory object is queried for the availability of a part
- the inventory object reports true to
teller, a callback object
The eventual send to the inventory object has two log entries: a Sent and its corresponding Got.
"class" : [ "org.ref_send.log.Sent", "org.ref_send.log.Event" ],
"anchor" : {
"number" : 3,
"turn" : {
"loop" : "http://localhost:8080/-/buyer/",
"number" : 3
}
},
"message" : "lqhjpwbbeemozk-2-0",
"timestamp" : 1274238401772,
"trace" : {
"calls" : [ {
"name" : "Main.Buy.fulfill",
"source" : "org/waterken/purchase_ajax/Main.java",
"span" : [ [ 68 ] ]
}, {
"name" : "Main.Buy.fulfill",
"source" : "org/waterken/purchase_ajax/Main.java",
"span" : [ [ 48 ] ]
} ]
}
The anchor uniquely identifies the origin of this message send as the 3rd messaging event from the buyer vat, turn 3.
The message is a generated string which uniquely identifies a message.
The trace is the stack capture at the point of the message send.
Note: The loop field identifies the vat by URI. By convention, Causeway picks up the part following "/-/", in this case buyer, for a short display name.
Note: The timestamp field is optional. Currently, it is ignored by Causeway, so it's not shown in the remaining trace records.
"class" : [ "org.ref_send.log.Got", "org.ref_send.log.Event" ],
"anchor" : {
"number" : 1,
"turn" : {
"loop" : "http://localhost:8080/-/buyer/product/",
"number" : 2
}
},
"message" : "lqhjpwbbeemozk-2-0",
"trace" : {
"calls" : [ {
"name" : "InventoryMaker.InventoryX.partInStock",
"source" : "org/waterken/purchase_ajax/InventoryMaker.java"
} ]
}
The corresponding Got record matches on message. The message delivery in the product vat starts a new turn, turn 2. Being at the top of a new turn, there is limited stack capture and getting a source span through Java reflection, is not practical.
Reporting true to teller has two log entries: a Sent and its corresponding Got.
"class" : [ "org.ref_send.log.Sent", "org.ref_send.log.Event" ],
"anchor" : {
"number" : 2,
"turn" : {
"loop" : "http://localhost:8080/-/buyer/product/",
"number" : 2
}
},
"message" : "ewrigzpctikrhk-1-0",
"trace" : {
"calls" : [ {
"name" : "InventoryMaker.InventoryX.partInStock",
"source" : "org/waterken/purchase_ajax/InventoryMaker.java",
"span" : [ [ 19 ] ]
} ]
}
The anchor uniquely identifies the origin of this message send as the 2nd messaging event from the product vat, turn 2.
"class" : [ "org.ref_send.log.Got", "org.ref_send.log.Event" ],
"anchor" : {
"number" : 1,
"turn" : {
"loop" : "http://localhost:8080/-/buyer/",
"number" : 10
}
},
"message" : "ewrigzpctikrhk-1-0",
"trace" : {
"calls" : [ {
"name" : "AsyncAnd.run",
"source" : "org/waterken/purchase_ajax/AsyncAnd.java"
} ]
}
The corresponding Got record matches on message. The message delivery in the buyer vat starts a new turn, turn 10.
Logging Promise-based Messaging in Waterken
(Note: This section on promises is incomplete and will be updated soon. tstanley 5/24/10)
Promise<Boolean> partP = _._(inventory).partInStock(partNo);
Promise<Boolean> creditP = _._(creditBureau).checkCredit(name);
Promise<Boolean> deliverP = _._(shipper).canDeliver(profile);
final Promise<Boolean> allOkP =
new AsyncAnd(_).run(partP, creditP, deliverP);
/*
* Register a when-block on promise returned by AsyncAnd.
* The block executes when the promise resolves (either
* fulfilled or rejected).
*/
_.when(allOkP, checkAnswers(_, inventory));
public Promise<Boolean> run(Promise<Boolean>... answers) {
final Channel<Boolean> result = _.defer();
final int[] expected = {answers.length};
for (Promise<Boolean> answerP : answers) {
/*
* Register a when-block on each promise. The block executes
* when the promise resolves (either fulfilled or rejected).
*/
_.when(answerP, new DoAnswer(expected, result.resolver));
}
return result.promise;
}
We start with a detailed description of a single message, partInStock. Then we'll look at how AsyncAnd collects the answers and reports a final result.
- The graph structure representing the message send and resolution
- The events logged
- The graph filter
public Void fulfill(Boolean answer) {
if (answer) {
myExpected[0]--;
if (myExpected[0] == 0) {
/*
* Resolve the promise with true.
*/
myResolver.apply(true);
} else {
/*
* Progress had been made in resolving the promise.
* If logging is on, a Progressed event record is written.
* If logging is off, this is a noop.
*/
myResolver.progress();
}
} else {
/*
* Resolve the promise with false. Notice that this
* short-circuits the logic: any remaining expected answers
* are ignored.
*/
myResolver.apply(false);
}
}
"class" : [ "org.ref_send.log.Sent", "org.ref_send.log.Event" ],
"anchor" : {
"number" : 2,
"turn" : {
"loop" : "http://localhost:8080/-/buyer/",
"number" : 3
}
},
"message" : "jisyrc633lurdq-2-0",
"trace" : {
"calls" : [ {
"name" : "Buyer.Buy.fulfill",
"source" : "org/waterken/purchase_promise/Buyer.java",
"span" : [ [ 59 ] ]
}, {
"name" : "Buyer.Buy.fulfill",
"source" : "org/waterken/purchase_promise/Buyer.java",
"span" : [ [ 44 ] ]
} ]
}
"class" : [ "org.ref_send.log.Got", "org.ref_send.log.Event" ],
"anchor" : {
"number" : 1,
"turn" : {
"loop" : "http://localhost:8080/-/buyer/product/",
"number" : 2
}
},
"message" : "jisyrc633lurdq-2-0",
"trace" : {
"calls" : [ {
"name" : "InventoryMaker.InventoryX.partInStock",
"source" : "org/waterken/purchase_promise/InventoryMaker.java"
} ]
}
"class" : [ "org.ref_send.log.Returned", "org.ref_send.log.Sent", "org.ref_send.log.Event" ],
"anchor" : {
"number" : 2,
"turn" : {
"loop" : "http://localhost:8080/-/buyer/product/",
"number" : 2
}
},
"message" : "jisyrc633lurdq-2-0-return",
"class" : [ "org.ref_send.log.Got", "org.ref_send.log.Event" ],
"anchor" : {
"number" : 1,
"turn" : {
"loop" : "http://localhost:8080/-/buyer/",
"number" : 6
}
},
"message" : "jisyrc633lurdq-2-0-return",
"class" : [ "org.ref_send.log.Fulfilled", "org.ref_send.log.Resolved", "org.ref_send.log.Event" ],
"anchor" : {
"number" : 2,
"turn" : {
"loop" : "http://localhost:8080/-/buyer/",
"number" : 6
}
},
"condition" : "http://localhost:8080/-/buyer/#p3",
"class" : [ "org.ref_send.log.Got", "org.ref_send.log.Event" ],
"anchor" : {
"number" : 1,
"turn" : {
"loop" : "http://localhost:8080/-/buyer/",
"number" : 9
}
},
"message" : "http://localhost:8080/-/buyer/#w3",
"trace" : {
"calls" : [ {
"name" : "AsyncAnd.DoAnswer.fulfill",
"source" : "org/waterken/purchase_promise/AsyncAnd.java"
} ]
}
"class" : [ "org.ref_send.log.Progressed", "org.ref_send.log.Resolved", "org.ref_send.log.Event" ],
"anchor" : {
"number" : 2,
"turn" : {
"loop" : "http://localhost:8080/-/buyer/",
"number" : 9
}
},
"condition" : "http://localhost:8080/-/buyer/#p6",
"trace" : {
"calls" : [ {
"name" : "AsyncAnd.DoAnswer.fulfill",
"source" : "org/waterken/purchase_promise/AsyncAnd.java",
"span" : [ [ 52 ] ]
}, {
"name" : "AsyncAnd.DoAnswer.fulfill",
"source" : "org/waterken/purchase_promise/AsyncAnd.java",
"span" : [ [ 26 ] ]
} ]
}
"class" : [ "org.ref_send.log.Got", "org.ref_send.log.Event" ],
"anchor" : {
"number" : 1,
"turn" : {
"loop" : "http://localhost:8080/-/buyer/",
"number" : 15
}
},
"message" : "http://localhost:8080/-/buyer/#w6",
"trace" : {
"calls" : [ {
"name" : "Buyer.CheckAnswers.fulfill",
"source" : "org/waterken/purchase_promise/Buyer.java"
} ]
}
Consider the sequence of events shown below.
Note: For the purpose of describing the trace records the screenshot shows events that actually will be filtered by Causeway and do not appear in the message-order view.
The registration of when-blocks, logged as SentIf records, are filtered from the message-order view, as they don't contribute to the understanding of message flow. However, they do appear in process-order, as shown below.
"class" : [ "org.ref_send.log.SentIf", "org.ref_send.log.Sent", "org.ref_send.log.Event" ],
"anchor" : {
"number" : 8,
"turn" : {
"loop" : "http://localhost:8080/-/buyer/",
"number" : 3
}
},
"condition" : "http://localhost:8080/-/buyer/#p6",
"message" : "http://localhost:8080/-/buyer/#w6",
"trace" : {
"calls" : [ {
"name" : "Buyer.Buy.fulfill",
"source" : "org/waterken/purchase_promise/Buyer.java",
"span" : [ [ 72 ] ]
}, {
"name" : "Buyer.Buy.fulfill",
"source" : "org/waterken/purchase_promise/Buyer.java",
"span" : [ [ 44 ] ]
} ]
}
"class" : [ "org.ref_send.log.SentIf", "org.ref_send.log.Sent", "org.ref_send.log.Event" ],
"anchor" : {
"number" : 5,
"turn" : {
"loop" : "http://localhost:8080/-/buyer/",
"number" : 3
}
},
"condition" : "http://localhost:8080/-/buyer/#p3",
"message" : "http://localhost:8080/-/buyer/#w3",
"trace" : {
"calls" : [ {
"name" : "AsyncAnd.run",
"source" : "org/waterken/purchase_promise/AsyncAnd.java",
"span" : [ [ 87 ] ]
}, {
"name" : "Buyer.Buy.fulfill",
"source" : "org/waterken/purchase_promise/Buyer.java",
"span" : [ [ 63 ] ]
}, {
"name" : "Buyer.Buy.fulfill",
"source" : "org/waterken/purchase_promise/Buyer.java",
"span" : [ [ 44 ] ]
} ]
}
Performance Issues in Waterken
Due to the expense of stack capture in Java, tracing in Waterken incurs roughly, an order of magnitude performance penalty. If tracing is off, there is no penalty.
Waterken guarantees message order delivery and in addition, if a connection is dropped, there's enough information to know about partial success. For example, if 2 messages (msg1, msg2) are sent from vat A to vat B, they are guaranteed to be processed in the order sent. If the connection is dropped after msg1 is successfully sent, when the connection is re-established, it is known that only msg2 must be resent.
The identifiers used to support these guarantees are also used for tracing. The advantage of these multi-purpose identifiers is there is no overhead when tracing is off (i.e., unique message identifiers, just for tracing, are not sent out over the wire.)
Note: Resending a message after a connection is re-established can result in 2 identical Sent events being logged. Causeway notices when the event records are identical and ignores the duplicate.
