Technology Barriers To Moving Apps and Services to a PaaSLanguage/Runtime Characteristics: CF only supports languages that have a buildpack that run on Linux. Cloud Foundry currently supports Ubuntu Trusty and CentOS 6.5 on vSphere, AWS, OpenStack, and vCloud infrastructures.
Degree of Statefulness - Runtime state includes caching and Sessions. Coordination of state across peer instances impacts performance. Any application that requires persistent heavy data processing. Apps that require large-scale stateful connections to external services. Use of frameworks that persist client state on the server side like JSF. Apps that keep a large amount of critical runtime state within the JVM i.e. stateful apps are bad candidates to the PaaS. Ideally all persistent state should reside outside the app in a HA data persistence tier.
File System - Cloud app instances are ephemeral. Local file system storage is short-lived. Instances of the same app do NOT share a local file system.Avoid writing state to the local file system. Does the app rely on a persistent local filesystem or storage ?
Transactionality - 2PC Transactional commit does not work across multiple distributed REST services. Prefer BASE(Basically Available, Soft state, Eventually consistent) over ACID(Atomicity, Consistency, Isolation & Durability). Transaction managers typical write transaction logs to local file system and rely on persistent server identity - both of these are not available by design in CF. http://queue.acm.org/detail.cfm?id=1394128
Configuration - Configuration should live external to the deployed app. Config must come from the Environment via environment variables via external configuration server. Apps that have embedded property files or one war/ear file per environment require the refactoring of configuration code before running on the PaaS. We recommend one code base tracked in revision control across multiple deploys. Can you deploy to multiple environments with single codebase.
Web Frameworks : Does the app leverage Java EE or Spring Frameworks and if so are the libraries bundled within the app or come with the app server. If there is strong dependence on an application server like WebSphere to provide dependencies or if proprietary application server APIs are used then that forces the app to use a particular buildpack or worse refactoring to start embedding the libraries within the app.
Startup/Shutdown Characteristics: Prefer apps that start cleanly without a lot of upfront initialization, coordination and shutdown without the need for comprehensive cleanup of state.
Is a startup/shutdown sequence necessary for your app to run effectively. Avoid creating any process instances outside the staged runtime.
Scalability of the App - Does the app rely on X-axis(horizontal duplication, Scale by cloning), Y-axis (functional decomposition, scale by splitting different things) or Z-axis (data partitioning, scaling by splitting similar things) ?. Design app as one or more stateless processes enabling scale-out via process model.Which other dependent services needs to be scaled when the app is scaled ?
Dependencies - How are external dependencies wired into the app ? Are dependencies isolated and explicitly declared. http://microservices.io/articles/scalecube.html
Inbound Protocols - Only a single inbound port is open to an application. Only protocol supported by CF is HTTP/HTTPs/WebSocket. No other protocols like RMI or JMX will work (unless tunneled over HTTP) on the inbound side. HTTPS is terminated at the load balancer. App that relies on transport security at the server will need to be modified. Apps that hold persistent long running socket connections to the server will not work, with apps using WebSocket being the only exception. Any sort of direct peer-to-peer messaging or custom protocol will not work since the warden containers are configured for ingress only over HTTP.
Logging - Are there multiple log streams ? Is there a strong requirement on local persistence of the logs ? Treat Logs of the app as event streams. Prefer console based logging to file based logging. Configure app server logging to log to the console (stdout/stderr) and thereafter drain to a long-term log analytics solution. Logs written to the local file system in CF are not persisted.
Rollback - Immutable code with instant rollback capability. Can the app tolerate multiple versions ? Can the app be reverted to a previous version with a single action ? Does deployment of the app require orchestration of multiple steps.
Security - Does the app rely on network centric or app centric security model ? We recommend relying on on application Layer 7 security rather than network security based on Firewall rules. How are application users authenticated and authorized. Is a Federated Identity and Authorization solution in place ? In CF, outbound egress from the app is controlled by application security groups applied to the warden container. You will need to configure whitelist rules for the services bound and protocols used for outbound communication.
Application Size - cf push times out for apps bigger than 250MB. Staging of apps that exceed a certain size becomes a big drain on resources for network marshalling/unmarshalling as well as runtime memory footprint. Keeping the droplet smaller results in faster deployment. client_max_body_size can be used to changed the max upload size on the Cloud Controller.
Performance & Monitoring - Can the app exhaust CPU under load? Are there any concurrency bottlenecks ? Can the app be instrumented for metrics ? Does the app provide a health check/actuator support in production. Does the app leverage app correlation IDs for distributed requests & responses. Does the underlying platform provide basic monitoring(CPU, Disk, Memory) and recovery (Spawning of exited instances) and instant(manual/automated) scaling. Can the logging and admin operations be enabled via runtime switches aka curl requests ?
Developer Experience - Does app architecture and web frameworks facilitate local development and cloud production. Does the app ruin on runtimes that run equally locally and in the cloud. Does the app use (Spring) profiles to encapsulate conditional app. configuration.
Cloud Native Architecture - Does the app compensate for the fallacies of distributed computing by leveraging framework libraries like Netflix OSS and/or SpringCloud OSS ? Does the app leverage zero downtime practices of forward and backward schema compatibility for REST microservices and databases. Can one service be deployed independently of other services. Is lockstep deployment of physical war's needed to manifest a logical app ?
Data Tier Services/Databases - Any persistence employed by the app has to be synchronized across datacenters and availability zones. The data tier/ Stateful tier of the app has to be managed in lock-step with the stateless tier. Focus has to be put on live content and data migration along with CAP theorem issues. App code has to be forward and backward compatible with schema changes in the data tiers.
For more on the exact process i.e. the HOW of migrating apps to the Cloud Foundry check this Pivotal blog post from Josh Kruck http://blog.pivotal.io/pivotal-cloud-foundry/features/how-do-i-migrate-applications-to-pivotal-cloud-foundry