L21 scalability
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This document discusses scalability and distributed systems. It introduces the scale cube model for scaling applications horizontally across multiple servers and vertically by splitting functionality. Session state can be stored on clients, servers, or in a database. Distributed systems require coarse-grained interfaces to minimize remote calls. Eventual consistency relaxes ACID properties and guarantees availability over consistency through asynchronous replication. The CAP theorem states it is impossible to guarantee consistency, availability, and partition tolerance simultaneously in distributed systems.
L21 scalability
- 2. Agenda Evolution - where are we today? Requirements of 21st century web applications Session State Distribution Strategies Scale Cube Eventual Consistency – CAP Theorm Real World Example
- 3. Evolution 60s 70s 80s 90s 00s IBM Mainframes Limited layering or abstraction IBM, DEC Mini-computers Unix, VAX “Dumb” terminals Screens/Files PC, Intel, DOS, Mac, Unix, Windows Client/Server RMDB Windows Internet HTTP Web Browsers Web Applications RMDB Windows, Linux MacOS Browsers, Services Domain Applications RMDB 10s iOS Android HTML5 Browsers Apps API Cloud NoSQL
- 4. Motivation Requirements of 21st century web systems – High availability – Millions of simultaneous users – Peak load of 1000s tx/sec Example – What if we need to handle load of 20.000 tx/sec? – That’s 1.2 million tx per minute
- 6. Business Transactions Transactions that expand more than one request – User is working with data before they are committed to the database • Example: User logs in, puts products in a shopping cart, buys, and logs out – Where do we keep the state between transactions?
- 7. State Server with state vs. stateless server – Stateful server must keep the state between requests Problem with stateful servers – Need more resources, limit scalability
- 8. Stateless Servers Stateless servers scale much better Use fewer resources Example: – View book information – Each request is separate REST was designed to be stateless
- 9. Stateful Servers Stateful servers are the norm Not easy to get rid of them Problem: they take resources and cause server affinity Example: – 100 users make request every 10 second, each request takes 1 second – One stateful object per user – Object are Idle 90% of the time
- 10. Session State State that is relevant to a session – State used in business transactions and belong to a specific client – Data structure belonging to a client – May not be consistent until they are persisted Session is distinct from record data – Record data is a long-term persistent data in a database – Session state might en up as record data
- 11. EXCERISE Question: Where do you store the session?
- 12. Ways to Store Session State We have three players – The client using a web browser – The Server running the web application and domain – The database storing all the data
- 13. Ways to Store Session State Three basic choices – Client Session State – Server Session State – Database Session State
- 14. Client Session State Store session state on the client How It Works – Desktop applications can store the state in memory – Web solutions can store state in cookies, hide it in the web page, or use the URL – Data Transfer Object can be used – Session ID is the minimum client state – Works well with REST
- 15. Client Session State When to Use It – Works well if server is stateless – Maximal clustering and failover resiliency Drawbacks – Does not work well for large amount of data – Data gets lost if client crashes – Security issues
- 16. Server Session State Store session state on a server in a serialized form How It Works – Session Objects – data structures on the server keyed to session Id Format of data – Can be binary, objects or XML Where to store session – Memory, application server, file or local database
- 17. Server Session State Specific Implementations – HttpSession – Stateful Session Beans – EJB When to Use It – Simplicity, it is easy to store and receive data Drawbacks – Data can get lost if server goes down – Clustering and session migration becomes difficult – Space complexity (memory of server) – Inactive sessions need to be cleaned up
- 18. Database Session State Store session data as committed data in the database How It Works – Session State stored in the database – Can be stored as temporary data to distinguish from committed record data Pending session data – Pending session data might violate integrity rules – Use of pending field or pending tables • When pending session data becomes record data it is save in the real tables
- 19. Database Session State When to Use It – Improved scalability – easy to add servers – Works well in clusters – Data is persisted, even if data centre goes down Drawbacks – Database becomes a bottleneck – Need of clean up procedure of pending data that did not become record data – user just left
- 20. What about dead sessions? Client session – Not our problem Server session – Web servers will send inactive message upon timeout Database session – Need to be clean up – Retention routines
- 21. Caching Caching is temporary data that is kept in memory between requests for performance reasons – Not session data – Can be thrown away and retrieved any time Saves the round-trip to the database Can become stale or old and out-dated – Distributed caching (message driven cache) is one way to solve that
- 22. Practical Example Client session – For preferences, user selections Server session – Used for browsing and caching – Logged in customer Database – “Legal” session – Stored, trackable, need to survive between sessions
- 23. QUIZ We are building an application for processing development grants. The application is complicated and users can login any time and continue work on their application. What design pattern would we use for storing the session? A) Client Session State B) Server Session State C) Database Session State D) No state required
- 24. QUIZ We are building an application for processing development grants. The application is complicated and users can login any time and continue work on their application. What design pattern would we use for storing the session? A) Client Session State B) Server Session State C) Database Session State D) No state required ✔
- 26. Distributed Architecture Distribute processing by placing objects on different nodes
- 27. Distributed Architecture Distribute processing by placing objects on different nodes Benefits – Load is distributed between different nodes giving overall better performance – It is easy to add new nodes – Middleware products make calls between nodes transparent But is this true?
- 28. Distributed Architecture Distribute processing by placing objects different nodes “This design sucks like an inverted hurricane” – Fowler Fowler’s First Law of Distributed Object Design: Don't Distribute your objects!
- 29. Remote and Local Interfaces Local calls – Calls between components on the same node are local Remote calls – Calls between components on different machines are remote Objects Oriented programming – Promotes fine-grained objects
- 30. Remote and Local Interfaces Local call within a process is very, very fast Remote call between two processes is order-of-magnitude s l o w e r – Marshalling and un-marshalling of objects – Data transfer over the network With fine-grained object oriented design, remote components can kill performance Example – Address object has get and set method for each member, city, street, and so on – Will result in many remote calls
- 31. Remote and Local Interfaces With distributed architectures, interfaces must be course-grained – Minimizing remote function calls Example – Instead of having getters and setters for each field, bulk assessors are used
- 32. Distributed Architecture Better distribution model (X scaling) – Load Balancing or Clustering the application involves putting several copies of the same application on different nodes
- 33. Where You Have to Distribute As architect, try to eliminate as many remote call as possible – If this cannot be archived choose carefully where the distribution boundaries lay Distribution Boundaries – Client/Server – Server/Database – Web Server/Application Server – Separation due to vendor differences – There might be some genuine reason
- 34. Optimizing Remote Calls We know remote calls are expensive How can we minimize the cost of remote calls? The overhead is – Marshaling or serializing data – Network transfer Put as enough data into the call – Course grained call – Use binary protocols – avoid XML
- 35. The Right Balance In Service Architecture, we want to split by functionality (Y Scaling) – Boundaries must be well designed – objects that work together are grouped together – APIs must be
- 36. The Scale Cube
- 37. Scaling the application Today’s web sites must handle multiple simulations users Examples: – All web based apps must handle several users – mbl.is handles >200.000 users/day – Betware must handle up to 100.000 simultaneous users and 1,2 million tx/min for terminal system peak load
- 39. The World we Live in Average number of tweets per day 500 million Total number of minutes spent on Facebook each month 700 billion SnapChat has 100 million daily active users who send 1 billion snaps each day Instagram has over 200 million users on the platform who send 60 million photos per day
- 40. Scalability Scalability is the ability of a system, network, or process to handle a growing amount of work in a capable manner or its ability to be enlarged to accommodate that growth With more load, how does the load of the system vary?
- 41. Scalability Scalability is the measure of how adding resource (usually hardware) affects the performance – Vertical scalability (up) – increase server power – Horizontal scalability (out) – increase the servers Session migration – Move the session for one server to another Server affinity – Keep the session on one server and make the client always use the same server
- 42. Scalability How is the system growth pattern – what is the formula?
- 43. Amdahl’s Law This law is used to find the maximum expected improvement to an overall system when only part of the system is improved In parallel computing, it states that a small portion of the program which cannot be parallelized will limit the overall speed-up available from parallelization
- 44. Amdahl’s Law Amdahl’s law for overall speedup 1 Overall speedup = F (1 – F) + S F = The fraction enhanced S = The speedup of the enhanced fraction If we make 20% of the program be 10x faster F=0.2 S=10 1 overall speedup = 0.2 (1 – 0.2) + 10 Gives 1.22 in overall speedup IF S = 1000, overall speedup is 1.25
- 45. Amdahl’s Corollary Make the common case fast – Common case being defined as “most time consuming” 40% 10x faster => 1.5625 20% 100x faster => 1.2468
- 46. The Optimization Process There is only one way to test scalability: Measure – Find the bottleneck (the common case) – Hypothesize about improvement – Make optimization – change only one thing a time – Measure again and repeat
- 47. The Scaling Problem We need to handle number of request to our system There are two ways to scale: – Vertically or scale up: Add more capacity to your hardware, more memory for example – Horizontal or scale out: Add more machines
- 48. Scaling Up This is the traditional approach for many monolithic systems Use a big powerful system Pros: – Easy to do, easy to understand – One memory space and one database Cons: – Has very hard limits – Does not work for the 21st century requirements
- 49. The Scale Cube One Monolithic system X-Axis: Horizontal Duplication Multiple Services Clustered Single data lookups Many System Duplicated Y-Axis: Split by Functionality No splits No splits Splits by functionality
- 50. Scaling Out (X scaling) This can work for monolithic systems if the database requirements is not high Use a many machines and distribute the load – Have one big powerful database Pros: – Scales well – handles much more load – Shared database Cons: – Session management is a challenge – Database is a bottleneck
- 51. Load Distribution Use number of machines to handle requests Load Balancer directs all request to particular server – All requests in one session go to the same server – Server affinity Benefits – Load can be increased – Easy to add new pairs – Uptime is increased Drawbacks – Database is a bootleneck
- 52. Clustering With clustering, servers are connected together as they were a single computer – Request can be handled by any server – Sessions are stored on multiple servers – Servers can be added and removed any time Problem is with state – State in application servers reduces scalability – Clients become dependant on particular nodes
- 53. Clustering State Application functionality – Handle it yourself, but this is complicated, not worth the effort Shared resources – Well-known pattern (Database Session State) – Problem with bottlenecks limits scalablity Clustering Middleware – Several solutions, for example JBoss, Terracotta Clustering JVM or network – Low levels, transparent to applications
- 56. The Scale Cube One Monolithic system X-Axis: Horizontal Duplication Multiple Services Clustered Single data lookups Many System Duplicated Y-Axis: Split by Functionality No splits No splits Many Services Data/User splits Splits by functionality Near infinite scaling
- 58. Transactions Transaction is a bounded sequence of work – Both start and finish is well defined – Transaction must complete on an all-or-nothing basis All resources are in consistent state before and after the transaction Example: Database transaction – Withdraw data from account – Buy the product – Update stock information Transactions must have ACID properties
- 59. ACID properties Atomicity – All steps are completed successfully – or rolled back Consistency – Data is consistent at the start and the end of the transaction Isolation – Transaction is not visible to any other until that transaction commits successfully Durability – Any results of a committed transaction must be made permanent
- 60. Transactional Resources Anything that is transactional – Use transaction to control concurrency – Databases, printers, message queues Transaction must be as short as possible – Provides greatest throughput – Should not span multiple requests – Long transactions span multiple request
- 61. Transaction Isolations and Liveness Transactions lock tables (or resources) – Need to provide isolation to guarantee correctness – Liveness suffers – We need to control isolation Serializable Transactions – Full isolation – Transactions are executed serially, one after the other – Benefits: Guarantees correctness – Drawbacks: Can seriously damage liveness and performance
- 62. Isolation Level Problems can be controlled by setting the isolation level – We don’t want to lock table since it reduces performance – Solution is to use as low isolation as possible while keeping correctness
- 63. Problem Serialization crates scalability bottlenecks Applications that support fully secure serialization of using RMDB have hard time with scale Can we scarify something? – Can we relax these requirements?
- 64. CAP Theorem States that it is impossible for a distributed computer system to simultaneously provide all three of the following guarantees: – Consistency: all nodes see the same data at the same time – Availability: a guarantee that every request receives a response about whether it was successful or failed – Partition tolerance: the system continues to operate despite arbitrary message loss or failure of part of the system
- 66. ACID vs. BASE BASE: Basically Available, Soft state, Eventual consistency Basically Available: Guarantees availability of the database Soft state: The state of the system can change over time - even without input. Eventual consistency: The system will eventually become consistent over time given no new input
- 67. ACID vs. BASE The difference has more to do with synchronous and asynchronous messaging For large scale systems asynchronous caters for the fastest and least restricted workflow
- 68. Asynchronous Eventual Consistency example Web Layer Requests Approve RMDB Msg Q Process
- 69. Measuring Scalability The only meaningful way to know about system’s performance is to measure it Performance Tools can help this process – Give indication of scalability – Identify bottlenecks
- 72. Real World Examples: Betware Iceland Data Center
- 73. ISP1 ISP2 Hardware firewall Load balancer 16 port 2Gbps SAN switch QLogic 12 x 300GB SAS 15K 24 x 300GB SAS 15K IBM Blade Chassis System DB CMS DB Backup Software Pair of each server on separate blade
- 77. Summary Requirements of 21st century web applications – Availability, Eventual consistency Session State – Client, Server, Database Distribution Strategies – Don’t distribute fine grained object – identify bouneries The Scale Cube Eventual Consistency – CAP Theorm Real World Example