Middleware Design

1. Middleware Design • Goals • identify the issues for middleware design in wireless and mobile environments • An illustrative middleware framework • Detailed design for an image transcoding proxy and application session handoff

2. Middleware Definition • RFC2768: • Def 1: those services found above the transport (I.e. over TCP/IP) layer set of services but below the application environment (i.e., below application-level APIs) • Def 2: a reusable, expandable set of services and functions that are commonly needed by many applications to function well in a networked environment. • Industry usage: • Software gateway (“glue”) between two apps

3. Issues for Middleware Design • Legacy systems and protocols • Diverse networks (wireline, indoor & outdoor wireless) • Network dynamics: congestion, link errors, failures, attacks • Device and platform heterogeneity • User mobility • Thin-client support • Large number of users and devices

4. Some middleware design goalsfor wireless and mobile devices • Improve user experience across heterogeneous devices (e.g. PDAs, laptops, desktops) • e.g. transcoding (adaptive content delivery) • Provide new services for heterogeneous devices • e.g. application state migration • Minimal change to the existing infrastructure and applications: may add/change a few more “boxes” • Adaptation to network dynamics (induced by mobility and wireless links) • Scalable and secure service • Service availability (in the presence of failures, attacks and large user population)

5. Transcoding middleware service • Client variations along 3 dimensions: • Network variation • bandwidth, latency and error behavior • Hardware variation • screen size/resolution, color/grayscale bit depth, memory, CPU • Software variation • Applications for specific MIME types (PDF, PS, PPT, AVI, etc.) • Codecs for specific encodings (H263, JPEG, etc.) • Transcoding goals: • Reduce latency experienced by user • Reduce image’s color depth, resolution to get smaller file • Provide access to new types of content • PDF  text, Speech  text

6. Transcoding design issues • Design issues for adapting to variation: • How: Datatype-specific lossy compression mechanisms: distillation & refinement based on (MIME) type of data • Where: at the content server or at a proxy • When: static or on-demand

7. Distillation and refinement • Main idea: high-level semantic types (MIME types) dictate datatype-specific operations • Images: can discard color info, high-frequency components, or pixel resolution • Video: additionally include frame rate reduction • Formatted text: can discard some formatting information • Datatype-specific distillation: highly lossy, datatype-specific compression that preserves most of the semantic content of a data object while adhering to a particular set of constraints • Datatype-specific refinement: fetching some part (possibly all) of a source object at increased quality, possibly the original representation

8. Choices to handle client variations • Server ignores variations: • low-end clients may suffer • Server use the most basic types & minimal graphics: • high-end client suffers • Servers provide multiple formats: • used today by major websites (ESPN, Amazon, Yahoo) • need to categorize clients into discrete classes • Progressive encodings: • typically assume that all parts of the encoded documents are equally important • On-demand distillation and refinement: • generate on-the-fly based on client characteristics

9. An Adaptive-Proxy Based Middleware Design Framework • Three-tier model: client – proxy – server • A programming model for proxy-based design: TACC • Transformation: distillation, filtering, format conversion, etc. • Aggregation: collect and collate data from various sources • Caching: both original and transformed content • Customization: user-customized service (user profiling, adaptive service to each user’s needs or device characteristics)

10. Why do we need a proxy ? • Advantages for servers: • Servers concentrate on serving high quality content, rather than having to keep multiple versions • Servers do not pay the costs required to do on-demand distillation • Advantages for clients: • Low-end clients can rely on the proxy to optimize content from servers designed for high-end clients • Client communicates with a single logical entity—proxy, allowing the client to manage bandwidth at the application level • Advantages for both: • Pushing the complexity away from both clients and servers by relocating it into the network infrastructure • Distillation and refinement can be offered as a value-added service by a service provider

11. A Scalable Cluster-based Infrastructure • Address three issues: incremental scalability, 24X7 availability, and cost effectiveness • A cluster based architecture for scalable network services • Exploit the strength of cluster computing • Cluster-based servers • BASE data semantics: basically available, soft state, eventual consistency.

12. Front-end Load-balancer Webserver Workstation cluster Cluster architecture

13. Why do we need clusters? • Scalability: • well-suited for networking service workloads that are highly parallel • Clusters can grow incrementally over time • High availability: • Natural redundancy due to the independence of the nodes • Hot upgrade: disable a node and upgrade it in place • Commodity building blocks: • Use low-end, high-volume PCs rather than high-end, low-volume machines • Bad thing about clusters: • Administration; component and system replication (software should decompose into loosely coupled modules); partial failures; shared state

14. An example: adaptive transcoding proxy • Web server  transcoding proxy  web browser • Proxy architecture: • Content analysis • Adaptive transcoding policies: when and how much to transcode • Transformation modules: text modification, images decode & compress • Key design goal: • Improve latency experienced by user at heterogeneous devices • fixed quality or fixed delay

15. Design • Two scenarios: • Store-and-forward image transcoding • Streamed image transcoding • Two main issues: • Whether to transcode • How much to transcode

16. How to decide whether to transcode? • Dp = transcoding delay, S = orig size, Sp = transcoded size • w/o transcoding: • 2*RTTpc + 2*RTTsp + S / min(Bpc, Bsp) • w/transcoding: • 2*RTTpc + 2*RTTsp + Dp + S / Bsp + Sp/Bpc • If Bpc < Bsp, proxy-based transcoding useful when: • Dp + S/Bsp < (S-Sp)/Bpc • How to predict transcoding delay? Bpc Bsp server proxy client

17. Details for store-and-forward image transcoding • Prediction • Transcoded image’s output size in bytes: high correlation between output size and the image area (number of pixels)  linear interpolation • Prediction of transcoding delay: approximated by linear function of the input image area • Policies: • Fixed-quality transcoder: if (transcoding = feasible), transcode according to user’s parameter vector • Fixed-delay transcoder: if(transcoding=feasible), search space of transcoding parameters to find optimal set that maximizes quality subject to the given response time, transcode using the optimal parameters

18. Transcoding internal stages • Determine target parameters • In-band or out-of-band data • Use HTTP headers • Use a client profile and/or network conditions • Download data and characterize it • E.g. get image’s type, resolution, and color depth • Apply heuristics and policies • How to match data’s characteristics to target parameters? • Multi-dimensional constraint satisfaction • Execute the transcoding • Typically can use off-the-shelf software

19. Streamed image transcoding • Perform transcoding under two stability conditions: • No buffer overflow • Output transmission link is not saturated

20. Another middleware service:Application session handoff • We want continuous access to our data across these machines • Middleware software will integrate data across devices • for immediate access to information anytime, anywhere • Move applications across multiple computers

21. More application session handoff • Applications will have session state • discrete data • multimedia, streaming data • Application session handoff: application’s state will move automatically and seamlessly across devices • Data will be transcoded for each device

22. Broad view of system Application Server High Bandwidth Network Middleware Cluster Wireless Network Clients

23. Application session handoff in action Legacy Multimedia DBMS

24. Middleware design issues for ASH • Client must incorporate application-layer library code to participate with proxy • Protocol gateway • client  proxy : custom control protocol + application-specific protocol • Proxy  server: HTTP, SMTP, RTP, etc. • Service discovery • Data consistency protocols • Scalability across cluster of proxies • PKI-based security

25. Summary • Middleware provides improved user experience or additional functionality • Middleware runs within limits of existing legacy system or protocols • New functionality typically implemented at a proxy • Clustering provides scalability for proxy services

27. Goals for Middleware Design • Minimal change to the existing infrastructure and applications: may add/change a few more “boxes” • Adaptation to network dynamics (induced by mobility and wireless links) • Support for heterogeneous devices (e.g. laptop, desktop, pocket PC, palm-devices) • Customized service (e.g., adaptive content delivery) • Scalable and secure service • Portability: seamless migration across computing platforms • User-friendly design • Service availability (in the presence of failures, attacks and large user population)

28. On-demand dynamic distillation • Issues to address: client variations along 3 dimensions: • Network variation: bandwidth, latency and error behavior • Hardware variation: screen size and resolution, color or grayscale bit depth, memory, CPU power • Software variation: application-level data encodings, etc. • Design principles for adapting to variation: • Datatype-specific lossy compression mechanisms: distillation and refinement based on semantic type of the data • On the fly adaptation: compute a desired representation of a typed object on demand • Complexity away from both clients and servers: done at an intermediate proxy

29. Sharing semantics • Traditional transactional database model: ACID (atomicity, consistency, isolation, and durability) • strongest semantics at the highest cost and complexity • No guarantee for availability • Suited for e-commerce transaction, billing users, maintaining user profile info etc. • Many users/services prefer availability rather than strong consistency or durability: • Stale data can be temporarily tolerated as long as all copies of data eventually reach consistency after a short time • Soft state: can be used to improve performance • Approximate answers are preferred if delivered quickly compared to exact but slow answer

30. BASE semantics • BASE: basically available, soft state, eventual consistency • Handle partial failures in clusters with less complexity and cost • Trading consistency for simplicity • Trading consistency for availability • Use of soft state to allow each watcher process to detect that its peer is alive (rather than mirroring the peer’s state), be able to restart its peer (rather than take over its peer’s duties)