Designing Large-scale Applications In Python

1y ago

29 Views

1 Downloads

898.15 KB

68 Pages

Report/dmca

Download PDF

Transcription

Speaker IntroductionMarc-André Lemburg–––––––––Python since 1993/1994Studied MathematicseGenix.com GmbHSenior Software ArchitectConsultant / TrainerPython Core DeveloperPython Software FoundationEuroPython SocietyBased in Düsseldorf, Germany(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 20152:68

Designing Python Applications(1/2) Python makes it very easy to write complexapplications with very little code– It’s easy to create bad designs fast– Rewriting code is fast as well(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 20155:68

Designing Python Applications(2/2) Application design becomes the most importantfactor in Python projects This talk presents a general approach to theproblem(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 20156:68

Large-scale applications What can be considered “large-scale” in Python ?– Server application: 100 thousand lines of Python code– Client application: 50 thousand lines of Python code– Third-Party code: 100 thousand lines of code– Typically a mix of Python code andC extensions(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 20157:68

Why write applications in Python ? (1/3) Highly efficient– Small teams can scale up against large companies– Very competitive turn-around times– Small investments can result in high gains(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 20158:68

Why write applications in Python ? (2/3) Very flexible– allows rapid design, refactoring and rollout– highly adaptive to new requirements andenvironments– no lock-in(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 20159:68

Why write applications in Python ? (3/3) Time-to-market– Develop / add new featuresin weeks rather than months– Be ahead of the game or– Stay competitive(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201510:68

High-level view Applications typically have to implement.– Customer interaction (user interface)– Information flow (application interface)– Decision process (business logic)– Accounting and data keeping (storage interface)(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201513:68

Think outside the box. Application design is in manyways like structuring a company:– Departments and divisionsneed to be set up– Responsibilities need to be defined– Processes need to be defined(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201514:68

The Design Concept(1/2) Structured approach to application design– Divide et Impera (divide and conquer)– Top-down method: Application modelProcessing modelLayer modelComponentsManagement objectsData and Task objects Lots of experience also helps (c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201515:68

The Design Concept(2/2)Zen of Application Design (from this import app)portapp.– Keep things as simple as possible,but not simpler (KISS).thisim– Before doing things twice,think twice (DRY).from– If things start to get too complex,management is needed.– If management doesn’t help,decomposition is needed.– Keep in mind: There’s beauty in design.(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201516:68

The Design Concept(2/2) Zen of Application Design (from this import app)– Keep things as simple as possible,but not simpler (KISS).– Before doing things twice,think twice (DRY).– If things start to get too complex,decomposition is needed.– If decomposition doesn’t help,management is needed.– Keep in mind: There’s beauty in design.(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201517:68

Divide et Impera: Step by step Goal: Break down complexity as far as possible ! Top-down method:––––––Application modelProcessing modelLayer modelComponentsManagement objectsData and Task objects(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201518:68

Start with the type of application Goal: Break down complexity as far as possible ! Top-down method:––––––Application modelProcessing modelLayer modelComponentsManagement objectsData and Task objects(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201519:68

Choose a suitable application model Client-Server– Client application / Server application– Web client / Server application Multi-threaded stand-alone– Stand-alone GUI application Single process– Command-line application– Batch job application .(c) 2015 eGenix.com GmbH, info@egenix.comClientServerGUIEvent HandlerBusiness LogicInputProcessorOutputPyWaw Summit 201520:68

How should requests be processed ? Goal: Break down complexity as far as possible ! Top-down method:––––––Application modelProcessing modelLayer modelComponentsManagement objectsData and Task objects(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201521:68

Identify the processing model Identify theprocessing scheme:Task 1Interface LogicTask 2––––Single processMultiple processesMultiple threadsAsynchronousprocessingServer LogicInterfaceApplicationLogicLogicTask 3StorageServerLogic gicLogicApplication LogicStorage Logic– A mix of the above(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201522:68

Identify the processing model Identify the process/thread boundaries:– Which components(need to) share the sameobject space ?ApplicationInterface LogicServer Logic– Where is state kept ?Application LogicStorage Logic– What defines anapplication instance ?(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201523:68

Break down by functionality Goal: Break down complexity as far as possible ! Top-down method:––––––Application modelProcessing modelLayer modelComponentsManagement objectsData and Task objects(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201524:68

Find the right layer model Every application can be dividedinto layers of functionalitydefined by the flow of datathrough the application– Top layer:interface to the outside world– Intermediate layers:administration and processingApplicationInterface LogicServer LogicApplication LogicStorage Logic– Bottom layer:data storage(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201525:68

Examples of layer modelsClient Client application:GUI / Application Logic / StorageLogic Web application:Web Browser/ Network / Web Server /Interface Logic (SCGI, WSGI) /Server Logic / Application Logic /Storage Logic Batch processing:File I/O / Application Logic /Storage LogicWebBrowserServerInterface LogicServer LogicApplication LogicStorage Logic Custom model(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201526:68

Example: Web Application Situation:– Client is a standard web-browser– Client will do lots of AJAX– Server needs to take a lot of load andwill have to do most of the calculation work– Server needs to be fail-safe– Server is connected to a database– Server needs to scale(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201528:68

Example: Web Application Solution:– Application BrowserWebBrowserWeb Server (Apache/nginx)– Processing model:multiple processmodel– Layer model:typical applicationserver layers(c) 2015 eGenix.com GmbH, info@egenix.comProcess Broker (WSGI)Server ProcessServer ProcessServer ProcessServer LogicServer LogicServer LogicApplication LogicApplication LogicApplication LogicStorage LogicStorage LogicStorage LogicInterface LogicInterface LogicInterface LogicDatabasePyWaw Summit 201529:68

Found the layer model: now what ? Layers are usuallyeasy to identify,given the applicationmodel but often hardto design(c) 2015 eGenix.com GmbH, info@egenix.comApplicationInterface LogicServer LogicApplication LogicStorage LogicPyWaw Summit 201530:68

Layers are still too complex Goal: Break down complexity as far as possible ! Top-down approach:––––––Application modelProcessing modelLayer modelComponentsManagement objectsData and Task objects(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201531:68

Break up layers into components Layers provide a data driven separation offunctionality Problem:– The level of complexity is usually too highto implement these in one piece of code Solution:– build layers using a set ofloosely coupled components(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201532:68

Component design Components should encapsulatehigher level conceptswithin the application Components provideindependent building blocksfor the application(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201533:68

Component examples Components ––––––––provide the database interfaceimplement the user managementimplement the session managementprovide caching facilitiesinterface to external data sourcesprovide error handling facilitiesenable logging managementetc.(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201534:68

Advantages of components(1/2) They should be easily replaceableto adapt the application to newrequirements, e.g.– porting to a new database backend,– using a new authentication mechanism, etc. If implemented correctly,they will even allow switching toa different processing model,should the need arise.(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201535:68

Advantages of components(2/2) Loose coupling of the components makes itpossible to– refine the overall application design,– refactor parts of the layer logic, or– add new layerswithout having to rewrite large partsof the application code(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201536:68

Component implementation Each component is represented by acomponent object Component interfaces must besimple and high-level enough to allow forloose coupling– Internal parts of the components are never accesseddirectly, only via the component interface Component objects shouldnever keep state across requests– Ideally, they should also be thread-safe(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201537:68

The Big PictureProcess Boundary (Multiple Process Model)Interface LayerRequestComponentApplication Instance LayerResponseComponentSystemComponentServer LayerSessionComponentUserComponentApplication omponentStorage LayerDatabaseComponent(c) 2015 eGenix.com GmbH, Waw Summit 201538:68

The Big PictureProcess Boundary (Multiple Process Model)Interface LayerRequestComponentApplication Instance LayerResponseComponentSystemComponentServer LayerSessionComponentUserComponentApplication LayerHandlerComponentAll Component Objectsare connected to ogComponentStorage LayerDatabaseComponent(c) 2015 eGenix.com GmbH, Waw Summit 201539:68

The special System Object One system component object whichrepresents the application instance– All component objects arecreated and managed by thesystem object– Components can access eachother through the system object(circular references !)– There can be multiple systemobjects, e.g. one running ineach thread(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201540:68

Components: Summary General approach:– Onesystem componentthat manages theapplication instanceProcess Boundary (Multiple Process Model)Application Instance LayerInterface entServer LayerSessionComponentUserComponentApplication Layer– At leastone componentper layer(c) 2015 eGenix.com GmbH, ponentLogComponentStorage onentPyWaw Summit 201541:68

Components too complex as well ? Goal: Break down complexity as far as possible ! Top-down approach:––––––Application modelProcessing modelLayer modelComponentsManagement objectsData and Task objects(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201542:68

Add management objects Management objects––––help simplify component object implementationswork on or with groups of low-level data/task objectsprovide application internal APIsinterface to the “outside world”,e.g. file system, database, GUI, etc.Note:The distinction between management objects andcomponent objects is not always clear (c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201543:68

Management object or component ? Use component objects to representlogical units / concepts within the application– without going into too much detail Use management objects to work oncollections of data/task objects– to simplify component implementations– to avoid direct interfacing between thedata/task objectsTry to never mix responsibilities(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201544:68

Divide et Impera: The Lowest Level Goal: Break down complexity as far as possible ! Top-down approach:––––––Application modelProcessing modelLayer modelComponentsManagement objectsData and Task objects(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201545:68

Lowest level: Data and task objectsData objects– encapsulate data (nothing much new here)Task objects–––––interaction with multiple objectsI/O on collections of objectsdelegating work to other management objectsinterfacing to component objectsetc.(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201546:68

Example: Internal CommunicationSystemComponentObjectAccess PathApplication erExportManagerCSVImport XMLImport XLSImportCSVExport XMLExport XLSExport(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201547:68

The Request Context Data Object This is useful for task based applications, e.g.web applications Data management:– Componentsdon’t storeper-request state !Process Boundary (Multiple Process Model)Interface LayerRequestComponentApplication Instance LayerResponseComponentSystemComponentServer LayerSessionComponentApplication LayerUserComponentRequest nent– Per-request datais stored and passedaround viaRequest Context Objects(c) 2015 eGenix.com GmbH, info@egenix.comLogComponentStorage onentPyWaw Summit 201548:68

Thinking outside the box. a recap Application design is in manyways like structuring a company:– Departments and divisions need to be set up(layer and component objects)– Responsibilities need to be defined(management vs. data/task objects)– Processes need to be defined(component/management object APIs)(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201550:68

Conclusion Structured application design can go a long way Divide-et-impera helps keep basic buildingsblocks manageable Complex doesn't have to be complicated(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201551:68

Structuring your modules First some notes on the import statement:– Keep import dependencies low;avoid “from import *”– Always use absolute import paths(defeats pickle problems among other things)– Always layout your application modulesusing Python packages– Import loops can be nasty;import on demand can sometimes help(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201553:68

Finding the right package structure (1/2) Use one module per– management/component class– group of object classesmanaged by the same management class Keep modules small;if in doubt, split at class boundaries(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201554:68

Finding the right package structure (2/2) Group components and associated managementmodules in Python packages Use the application and layer model as basis forthe package layout(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201555:68

Data, classes and methods Use data objects for data encapsulation – instead of simple types(tuples, lists, dictionaries, etc.) Namespace objects are onehonking great idea, let's do more of those (c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201556:68

Data, classes and methods Use methods even for simple tasks – but don’t make them too simple Use method groups for more complex tasks /APIs– e.g. to implement a storage query interface(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201557:68

Data, classes and methods Use mix-in classes if method groups can bedeployed in more than class context If you need to write the same logic twice,think about creating a mix-in class toencapsulate it, or put it on a base class Avoid using mix-in classes, if onlyone class makes use of them(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201558:68

Make mistakes and learn from them If an implementation gets too complicated,sit down and reconsider the design – often enough a small change in the wayobjects interact can do wonders– regroup functionality– add more methods Magic word: Refactoring(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201559:68

Refactoring Be daring when it comes to rewritinglarger parts of code !– It sometimes takes more than just afew changes to get a design right– It is often faster to implement a good designfrom scratch, than trying to fix a broken one(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201560:68

Often forgotten: Documentation Always document the codethat you write ! Use doc-strings and inline comments– doc-strings represent your method’scontracts with the outside world Block logical units using empty lines – Python loves whitespace (c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201561:68

Often forgotten: Documentation Document the intent of the methods, classes andlogical code units – not only their interface– and write tests as functional documentation Use descriptive identifier names – even if they take longer to type(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201562:68

Quality Assurance: XP helps ! Try to use some extreme programming techniqueswhenever possible Always read the code top to bottom:– after you have made changes oradded something new to it– try to follow the flow of informationin your mind (before actually running the code) Write unit tests for the code and/or test ituntil everything works as advertised in thedoc-strings(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201563:68

Quality Assurance: More tips Typos can easily go unnoticed in Python:– use the editor’s auto-completion functionas often as possible– Use tools like PyLint to find hidden typos andpossibly bugs Always test code before committing it tothe software repository(c) 2015 eGenix.com GmbH, info@egenix.comPyWaw Summit 201564:68

ContacteGenix.com Software, Skills and Services GmbHMarc-André LemburgPastor-Löh-Str. 48D-40764 LangenfeldGermanyeMail:Phone:Fax:Web:(c) 2015 eGenix.com GmbH, info@egenix.commal@egenix.com 49 211 9304112 49 211 3005250http://www.egenix.com/PyWaw Summit 201568:68

What can be considered "large-scale" in Python ? - Server application: 100 thousand lines of Python code - Client application: 50 thousand lines of Python code - Third-Party code: 100 thousand lines of code - Typically a mix of Python code and C extensions