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Implementing MLP

Implementing MLP. speaker. Thomas Lindgren / Fredrik Linder / Magnus Eklund CellPoint AB. Location services. Distinctive feature of mobile services Measurements from network collected in a location server (spec: 03.71) Clients access information via HTTP/XML interface to server

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Implementing MLP

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  1. Implementing MLP speaker Thomas Lindgren / Fredrik Linder / Magnus Eklund CellPoint AB

  2. Location services • Distinctive feature of mobile services • Measurements from network collected in a location server (spec: 03.71) • Clients access information via HTTP/XML interface to server • Clients are portals, resellers, operators, … which then provide info to end-users

  3. Standardization • Standardized info format from mobile net => location server • Plethora of protocols for accessing info location server => client • Location interoperability forum (LIF) founded to bring some order • Equipment manufacturers, operators, … • Mobile Location Protocol (MLP) = unifier

  4. The MLP Protocol • Mobile Location Protocol 3.0.0 based on HTTP/1.1 and XML • About half a dozen DTDs define data • Many features • Desired accuracy, max age, … • Presentation coordinate system • Varying geometric shapes in reply • Multitude of data formats • ”Every feature is optional”

  5. SIMPL2 and MLP • SIMPL2 developed by Cellpoint as an MLP compatible protocol • Removed some features • Zoning, triggering • Still had to signal that features not supported • Added some features • CLPT-specific extensions for charging records • Many changes in MLP and SIMPL2 during development • Fourteen SIMPL2 draft specifications prior to release • Seven released during May-June

  6. SIMPL2 request • <?xml version ="1.0" ?> • <!DOCTYPE svc_init SYSTEM "MLP_SVC_INIT_300.DTD"> • <svc_init ver="3.0.0"> • <hdr ver="3.0.0"> • <client> • <id>application_4</id> • <pwd>secret</pwd> • </client> • </hdr> • <slir ver="3.0.0"> • <msids> • <msid type="MSISDN">46702711038</msid> • </msids> • <geo_info> • <CoordinateReferenceSystem> • <Identifier> • <code>4326</code> • <codeSpace>www.epsg.org</codeSpace> • <edition>6.1</edition> • </Identifier> • </CoordinateReferenceSystem> • </geo_info> • </slir> • </svc_init>

  7. SIMPL2 reply • <?xml version ="1.0" ?> • <!DOCTYPE svc_result SYSTEM "MLP_SVC_RESULT_300.DTD"> • <svc_result ver="3.0.0"> • <slia ver="3.0.0"> • <pos> • <msid>46702711038</msid> • <pd> • <time utc_off="+0200">20020623134453</time> • <shape> • <CircularArea srsName="www.epsg.org#4326"> • <coord> • <X>20 30 5.4W</X> • <Y>0 0 3.5N</Y> • </coord> • <radius>570</radius> • </CircularArea> • </shape> • </pd> • </pos> • </slia> • </svc_result>

  8. Implementing MLP • Integrate inets and xmerl • Act as server (MLS) • Act as proxy (MLB) • Must still handle all of MLP for proxying non-CLPT location servers • Thoroughly validate all incoming data • Requests and replies • Translate external  internal format • Dispatch to other server or compute location

  9. The bad part • Aggressive dev schedule (5-6 months x 3 people) • Goal: Release soon after MLP specification is finalized • Specification mutates quickly • Major and minor syntax and feature changes • Data formats • Error codes • Specification bug fixes • Only protocol syntax specified, not semantics • Semantics sometimes unclear

  10. The first-half score • Unsatisfactory code • Specification changes => patch upon patch • Unsatisfactory testing • Hard for developers to test such a big protocol • Hard for QA to know what worked in a given release • Time spent on reacting to trouble reports and specification changes • Release date approaching, but not release

  11. Second-half kickoff • We needed to get bugs and changes under control • Code must become easy to change • Esp. XML validation • Code must have high quality before QA begins • Fixing bugs via QA too slow and unhappy for us and QA • Must quickly resolve arguments about TRs • Many specification changes => many arguments about what was valid => lost time

  12. Abstraction • Get rid of records in crucial places • Use abstract data types instead • Encapsulate data representation in module • Can check that data are consistent when created • Operations are named => legible code, intention clear • (Programming 101 …) • Why weren’t ADTs used already? • Record notation more convenient (e.g., pattern match) • Records already provide representation independence (sort of)

  13. Quick and dirty encapsulation #request_rec{misc = Priv, subscr_i=X#subscr_rec.f3} => request_adt:set_subscriber_info(Priv, X) -module(request_adt). set_subscriber_info(Priv, X) -> #request_req{misc=Priv, subscr_i=X#subscr_rec.f3}.

  14. XML validation • Original code: traverse xml tree, check values • Rewrite for fast change • Use a rule interpreter + separate validation rule set • Easy to check that rules obey current specification • Easy to rewrite or extend without intro bugs • Pace of development made this crucial • Many draft specs => each must be integrated quickly

  15. Validation rules -define(tag_specs, [{'svc_init', blank, [{'ver', {'or', [{member, ["3.0.0"]}, {unsupported, ver}]}}]}, ... ]). apply_rule(blank, Value) -> [] == strip_whites(Value); apply_rule({'or', Rules}, Value) -> lists:any(fun(Rule) -> apply_rule(Rule, Value) end, Rules); ...

  16. Improve quality • Go for (pre-QA) automated testing • Not a new idea, but awesomely useful (again) • Wrote tester from scratch • effort paid back very quickly • Exercise all protocol features • Regression test case added when TR appears • Detects integration bugs as well • SIMPL2 ”on top of food chain”

  17. Test case specification test_series(1, 1) -> Clients = [{"service_a", "secret", ?OK}, ...], MSID = "...", [ {Expect, ?svc_init(?hdr_client(Name, Pwd), ?slir(?msids(MSID), ?default_geo_info)) || {Name, Pwd, Expect} <- Clients ]; ...

  18. Social issues • Some XP practices used (see paper) • Pair programming worked well • More refactoring should have been done • Elegance is (should not be) optional • Hard to take the time

  19. Evaluation • Worked very well in this setting • Bug fixing reduced • Bug hunting reduced • Faster turnaround • We could direct efforts to the right areas • Resolve grey areas of SIMPL2 and MLP • Tighten code • Add test cases • ”Virtuous circle”

  20. Performance evaluation • Measured SIMPL2 by running test suite • Single request at a time on unloaded system • Varying cases rather than weighted to normal • Results: • 15% time in scanning and parsing • 8% in validation • 18% in port_command, port_close, … • (tests include acting as proxy or server, Oracle access, …) • So any speedup from optimizing validation is limited

  21. Future work • Extend data driven design • Rule-driven translation from/to internal format? • Generate a validating XML parser given a validation spec? • Extend xmerl

  22. Final score • ADTs > Records • Data driven design (= validation rules) reduce complexity • Automated testing • XP-practices (pair programming) • SIMPL2 released to customer same week as final MLP specification was released • Another grisprotokoll bites the dust

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