The planning system for site

On the level of abstraction of the subject area requirements for the site (web service) is, IMHO, not different from the specification for any application client-server. Session is already a mechanism for implementing these abstractions, and in different languages/frameworks/... they can be very differently implemented, or even hidden from the developer, which operates with users, access rights, etc.

in my opinion to describe in the TOR "make me a session layout `CREATE TABLE sessions....` and Potts layout `CREATE TABLE psto ...`" this is stupid and not necessary
you must have specific requirements that must be met, and HOW it's done — none of your business, you are not a professional in this business and why solve should you do not understand
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write that the site is calculated at minimum load such that the platform launch there for example Debian,PHP >5.3, MYSQL >5 apache, nginx ,memcached available
user registration is required, the user can fill in fields (....)
the user can leave comments, so...
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generally describe what you want to, and not as
also, there is the so-called code metrics, you can try to think about the restrictions on data metrics
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also if you can specify in the TOR stage design the internal API / logic and its coordination

How about you just give to outsource competent people? just find those who make on proven frameworks and describe just the functional requirements.

if you plan, outserts, sit down and write TK together. better writing TK to draw up a separate contract.
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ideally, find another consultant, an architect from the side and show him TK.

Analysis of customer requirements (including interviewing), business analysis, system analysis, modelirovanie subject area, etc. — stages of automation that must be performed before the formation of specification for the development of the program. For these jobs individuals are: system and business analysts, analysts, which is typically more expensive than software developers. Often, the developers themselves carry out these works, which can potentially cause harm: developers will bring to TK associated with gr that it is more profitable, for example they can implement, not what will bring the greatest benefit to the customer.
It is wiser to hire a separate specialist/firm for automation for analysis of requirements and creation of specification for development or to raise such in his team.