Causeway Platform Developer

From Erights

Latest revision as of 23:48, 16 June 2010

Instrumenting a Platform to Generate Causeway Trace Logs

Causeway supports debugging distributed programs built on the communicating event loops concurrency model. This document describes Causeway's language-neutral trace log format and explains how to instrument a platform to generate trace logs. For concreteness, we show the trace log records generated for a simple distributed program. We discuss three different versions of the purchase-order example program, starting with the simplest implementation: Simple Ajax-style (below). If your platform supports promises, continue with Ajax with promotion and full promises.

Getting Started

• See Causeway for user documentation, which includes instructions for launching Causeway from a command line shell.

• Enable Causeway's debug view. Setting Causeway's debug flag enables a debug view. As events are selected in the viewer, the debug view shows the corresponding trace record from 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.)

• Open the purchase-order example program in the Causeway viewer. The examples, under the Help menu, are written for the AmbientTalk and Waterken platforms and they implement different message-passing styles. Select Help >> Open Waterken Example (Simple Ajax-style).

(Note: Starting with one-way asynchronous messages, two patterns for handling responses are continuation-passing and promises. In continuation-passing a request carries a callback argument to which a response should be sent. We refer to this as Ajax-style messaging, as it is the dominant pattern for web application development. A promise (future, in AmbientTalk) is a place-holder for the answer to a query; when the answer becomes available the promise resolves to that value.)

• Read the source code. It's part of the E distribution and can be found here.

   e/src/esrc/scripts/test/causeway/waterken/sources
   e/src/esrc/scripts/test/causeway/ambientTalk/sources

The purchase-order example program implements a procedure for handling new purchase orders. Before an order is placed, certain conditions must be met: the item is in stock and available, the customer’s account is in good standing, and the delivery options are up to date. An object residing in the "buyer" process (or vat) has remote references to objects residing in the "product" and "accounts" vats. The buyer queries the remote objects with asynchronous (non-blocking) message sends. The answers from the asynchronous inquiries must be collected and examined. The order is placed only if all requirements are satisfied.

If instrumenting a platform, your first test case should be the purchase-order example, ported to run on your platform. Start with Ajax-style using callbacks. This approach is less expressive than promises, but generating the trace log is more straightforward.

Causeway's Trace Log Format

Causeway supports the trace log format defined by Tyler Close at waterken.sourceforge.net/debug/.

The simple Ajax-style example logs the event types indicated by the colored boxes.

When causality tracing is on the events are logged as follows.

Logging Ajax-style Messaging in Waterken

The code snippet below shows the 3 queries being fired off. All 3 answers must be examined before the order is placed. Collecting the answers is handled by an AsyncAnd object. To collect the three answers, teller is passed as an argument to each of the remote queries, serving as a callback function.

    Callback teller = new AsyncAnd(_, 3, checkAnswers(_, inventory));

    _._(inventory).partInStock(partNo, teller);
    _._(creditBureau).checkCredit(name, teller);
    _._(shipper).canDeliver(profile, teller);

Consider the sequence of events from the message-order view shown below.

1. a remote inventory object is queried for the availability of a part
2. 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.

In the Sent trace record:

anchor uniquely identifies the origin of this message send as the 2nd messaging event from the buyer vat, turn 3.

message is a generated string which uniquely identifies a message.

trace is the stack capture at the point of the message send.

The corresponding Got has a matching 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 yet supported.

(Note: The loop field identifies the vat by a unique string. By convention, Causeway picks up the part following "/-/", in this case buyer, for a short display name.)

(Note: The timestamp field is optional. Currently, Causeway ignores it, so it's not shown in the remaining trace records.)

    "class" : [ "org.ref_send.log.Sent", "org.ref_send.log.Event" ],
    "anchor" : {
      "number" : 2,
      "turn" : {
        "loop" : "http://localhost:8080/-/buyer/",
        "number" : 3
      }
    },
    "message" : "qvet6lrs4zsfpv-2-0",
    "timestamp" : 1275338022120,
    "trace" : {
      "calls" : [ {
          "name" : "Main.Buy.fulfill",
          "source" : "org/waterken/simple_purchase_ajax/Main.java",
          "span" : [ [ 68 ] ]
        }, {
          "name" : "Main.Buy.fulfill",
          "source" : "org/waterken/simple_purchase_ajax/Main.java",
          "span" : [ [ 48 ] ]
        } ]
    }

    "class" : [ "org.ref_send.log.Got", "org.ref_send.log.Event" ],
    "anchor" : {
      "number" : 1,
      "turn" : {
        "loop" : "http://localhost:8080/-/buyer/product/",
        "number" : 2
      }
    },
    "message" : "qvet6lrs4zsfpv-2-0",
    "timestamp" : 1275338022291,
    "trace" : {
      "calls" : [ {
          "name" : "InventoryMaker.InventoryX.partInStock",
          "source" : "org/waterken/simple_purchase_ajax/InventoryMaker.java"
        } ]
    }

Reporting true to teller has two log entries: a Sent and its corresponding Got.

The reply to the query is the 2nd messaging event from the product vat, turn 2.

The corresponding Got has a matching message. The message delivery in the buyer vat starts a new turn, turn 10.

    "class" : [ "org.ref_send.log.Sent", "org.ref_send.log.Event" ],
    "anchor" : {
      "number" : 2,
      "turn" : {
        "loop" : "http://localhost:8080/-/buyer/product/",
        "number" : 2
      }
    },
    "message" : "4qjohg533s6cjn-1-0",
    "trace" : {
      "calls" : [ {
          "name" : "InventoryMaker.InventoryX.partInStock",
          "source" : "org/waterken/simple_purchase_ajax/InventoryMaker.java",
          "span" : [ [ 19 ] ]
        } ]
    }

    "class" : [ "org.ref_send.log.Got", "org.ref_send.log.Event" ],
    "anchor" : {
      "number" : 1,
      "turn" : {
        "loop" : "http://localhost:8080/-/buyer/",
        "number" : 10
      }
    },
    "message" : "4qjohg533s6cjn-1-0",
    "trace" : {
      "calls" : [ {
          "name" : "AsyncAnd.run",
          "source" : "org/waterken/simple_purchase_ajax/AsyncAnd.java"
        } ]
    }

In the Causeway viewer, individual tree items represent events and their descriptive labels depend on the information available in the logged record for the event. To present the most useful information available, Causeway labels tree items according to the following priority.

1. User comments specified in the text field, prefixed with "#".
2. Source code specified in the top call stack object. The trace object must include a span.
3. Source file and function name specified in top call stack object. The trace object must include a name and source.
4. If none of the above, the label is a Causeway-generated comment, prefixed with "##", e.g., ## unknown sender.

The resulting process-order view and message-order view are shown below.